If there is any chance the answer in your own mind is “no, they might not sit still and move around” out of your reach/eyesight, pick a different cabin. You know you’re children better than anyone.
I wouldn’t ever consider it when my son was 2. He’s almost 4 now and I still wouldn’t really consider it. For FT, my son probably isn’t the most traveled, but he’s done around twenty 2-3 hour flights and two 7+ hour flights. He’s very well behaved (FA’s and others’ opinions/comments, not mine 😁
but the out of sight/reach factor is a no-go for me personally.
I was on a CRJ-200 flight today where a younger girl, probably around 10-12 years old, got up and walked back to the bathroom right before the landing gear came down on final. The FA didn’t stop her or say anything. This was abysmal to me as the the girl was in row 1 literally next to the FA. We landed less than 10 minutes later (probably sooner, but I’m being generous).
Could you imagine if the girl was walking back up the aisle when we landed? Why didn’t the FA stop her in the first place knowing we were about to land? Just some things to think about as a parent.