I can imagine no valid objection to a priest giving an impromptu blessing to the children. I, too, have seen that some priests give a blessing and others do not. I've never seen anyone complain either way.
Here are practices in the Novus Ordo that I do object to:
1. When I was still mired in the Novus Ordo, I never saw one of the ordinary laymen give any kind of blessing to young children or babies in arms when giving communion standing in the queue. However, I was in a parish that was run by a nun (who one wouldn't know was a nun just by looking at her, though she always wore a skirt and jacket that matched the color of the vestments) who would hold the communion host and "bless" people not receiving communion with the host.
2. In the Novus Ordo it is often encouraged that everyone go to communion (so as not to be conspicuous in remaining in the pew) but, if the person believes he should not receive communion for any reason, the person folds his arms across his or her chest (as opposed to holding his hands out to receive the wafer) and the priest will bless the individual. I've even seen people do this to a layman handing out communion who, rather embarrassingly, "blessed" the individual.