Involuntary distractions do not detract from the merit of saying the rosary.
Say it whenever of you can.

As one who has a large collection of small children ... it is a struggle. I'm being honest! We have it set for before dinner (advise of an old priest: this is what Mgr. Lefebvre always had in Econe). It doesn't always happen - even with the best of intentions. We will always pray it later, but there are sometimes lulls - this happens. 

I once had a priest tell me that he received some advise from a Franciscan from Morgan about mental prayer specifically, but about prayer in general: the most important thing about prayer is simply to be in front of God. Your efforts matter, and God sees the difficulty that children can bring to the family rosary ... as I remind my wife continually, St Louis de Montfort says in the Secret of the Rosary that as long as the distraction are involuntary (children during family rosary will be a distraction, even when sitting nicely and holding their beads!), there is no sin, the prayer is meritorious, and that the larger the distraction that you fight through, the more merit. Keep going mom. Even if you say a decade here and there while the baby sleeps, it is better than nothing. Remember, this too shall pass!