Today talking with my nephew (8 yr) I mentioned today is All Soul's day and that it's good to pray for the souls in purgatory, he mentioned he would pray for his grandfather who I am pretty certain was eastern orthodox. I kept my mouth shut but I feel like I should have mentioned that praying for non-catholics does them nothing and is a waste of prayer.. i think I was influenced by human respect.

Op here, i will add that I am quite scrupulous and that bis mothers side is Catholic (novus ordo) and fathers side is eastern orthodox.

You are scrupulous! No prayer is wasted, especially that of an innocent child. Do not complicate his life. Keep it simple. It is not your role to intrude on the parents’ domain.

Probably an imperfection or venial sin at best.  Does the nephew even know that the grandfather was Eastern Orthodox?  Besides, since the grandfather was validly baptized, it's always possible that he had a revelation or conversion on his deathbed and might be in need of prayer.  Now, the Church does not pray publicly for deceased non-Catholics, due to the danger of religious indifferentism, and denial of EENS dogma, but it's also a matter of prudence to determine how and when to curb any such tendency in this nephew.  You wouldn't just say, "Hey, sonny, gramps is burning in Hell."  That might cause them to be ill disposed toward the Catholic Church, so you have to be cautious, prudent, etc. in making such corrections, especially where it comes to a slightly more distant relative (in the case a nephew) whose thought patterns, temperament, and dispositions you are not intimately familiar with.

Excellent answer!

It's not quite that simple.  Church has in fact rebuked bishops who publicly asked the faithful to pray for non-Catholics, because it instills religious indifferentism and heresy in souls.  So it's nothing to trifle with and "no prayer is wasted" is not the issue.  It's not about whether the child's prayers are being wasted but about what doctrine is forming in his mind.  Orthodox Doctrine is essential.  Question is more along the lines of whether prudence dictates that a correction should be made, depending on whether the child would be receptive to it rather than making him resent the Church.

It is quite simple. The OP is a layperson speaking to a child, not a bishop addressing his faithful. There is no need to make a treatise on it. And the issue is certainly about "wasting" prayer.

I feel like I should have mentioned that praying for non-catholics does them nothing and is a waste of prayer.

Unless the OP has been charged with the child's religious formation, s/he really ought to refrain from commenting of the eternal fate of his grandfather, but he should be encouraged to pray for him regardless. If anything is to be said, it should be expressed in general terms rather than personal terms. Why introduce a child to one's own scruples. Obviously it is not a question of religious indifference on the part of the OP.

Today talking with my nephew (8 yr) I mentioned today is All Soul's day and that it's good to pray for the souls in purgatory, he mentioned he would pray for his grandfather who I am pretty certain was eastern orthodox. I kept my mouth shut but I feel like I should have mentioned that praying for non-catholics does them nothing and is a waste of prayer.. i think I was influenced by human respect.

I think you did well, both the prudent and the charitable thing. You don't want to stamp an 8 year old mind with thoughts it cannot possibly grapple with. You do want to stamp it with thoughts it can handle, namely, in this case, that it is a good and holy thing to pray for the dead.  

And while we're at it, you don't want to oppress your own mind with thoughts it cannot possibly grapple with - thoughts that come from beneath, yes?

So I didn't sin? Also not sure what you mean by the second part.

I'm no priest, but I think you did well. I think you acted with virtue and good sense. You were dealing with an 8 year old. The whole case resolves on that one point. It's about him, not you. It's the question of how to gift him with wisdom and charity. You owed him something in that exchange, because you are an elder in the Faith. You owed him example and instruction - and, I repeat, you gave what you owed, and did not err.

The second part relates to the possibility that you are scrupling over this. Scruples are temptations from the devil, who is from beneath.

Today talking with my nephew (8 yr) I mentioned today is All Soul's day and that it's good to pray for the souls in purgatory, he mentioned he would pray for his grandfather who I am pretty certain was eastern orthodox. I kept my mouth shut but I feel like I should have mentioned that praying for non-catholics does them nothing and is a waste of prayer.. i think I was influenced by human respect.

One other thought. I pray for dead non-Catholics the same way I pray for living ones. Prayer is outside of time. You can pray that they receive the grace of conversion at the moment of their death. You can pray this hours, days, years, even centuries after their death.

You'll think I'm nuts, but I pray for Pontius Pilate, the poorest slob this world has ever known. Talk about a victim of circuмstances. How many of us would have stood up to that onslaught of persecution? Yes, Pilate was persecuted by the Jews. And we know that his wife was touched by grace. Even our Lord encouraged him by telling him his sin was not as great as that of the Jews. 

I have deep, deep sympathy for Pilate, my countryman. I pray for him, that he repents and confesses the truth before he closes his eyes. I read once on Traditio that he is venerated as a Saint in the East. That gave me much happiness.  

Now, none of this is mental fodder for an 8 year old. All he requires for now is that it is a good and holy thing to pray for the dead. When he is older, he will be ready for distinctions and subtleties enough.

He is certainly old enough now to be taught a prayer or two in Latin, and how to practice daily devotions for the Poor Souls. So, you can focus on amping his praxis.   

Pax!