I've rarely seen small children on the upper deck and in fact, this is why I book the upper deck for night flights (in other words, please don't take your infant or toddler on the upper deck at night; daytime is a different story - when I want to be awake). I've often seen parents with babies and toddlers in the middle, back-facing seats of the last row on the lower deck - this seems to give the parents and kids a little privacy, some room to "operate" - and yet some decent access to bathrooms and aisles when they need to move around (they don't have to step over other people's footstools to get out).