I have, unfortunately, heard this a number of times after Mass. Such statements anger me, in large part because I think they reflect backward thinking about things. (I should note here that I try not to get angry with the person who says it, for I know they aren't intending to be hurtful at all. I should also note that I sit in the front so my children can see, on the advice of my former confessor, a very holy Benedictine monk.)
Like I said, I sit in the front. It can be a challenge at times, as Leo is at an age where he wants to explore, especially now that he's walking. He can sometimes get overly excited and let out a squeal (which would be very audible at any location in the sanctuary) and sometimes he has gallbladder attacks. I get up with him if necessary, leaving the other two (I am always where I can see them). Charlotte wants to follow me in those cases, but I try to get her to stay in the pew. My husband is in the choir. When I'm sitting in the pew, then both Kieran and Charlotte are usually joining in as they can, especially if we've gone over our expectations beforehand; Kieran of course can do as he should with or without me sitting there. I keep distractions to a minimum as much as possible, but some Masses go better than others in that regard. After all, they are young children who are sometimes tired or hungry or just having a hard time.
Now for why I think telling a family with young children to sit in the back to minimise distractions is a bit backward. An adult or teen can, presumably, continue to focus on the Mass even with children near. An adult or teen should know the Mass well enough to continue praying it even if he cannot hear everything. After all, if that adult or teen were to attend a Tridentine Mass, he wouldn't hear every word by design, since many of the prayers are said by the priest alone in a soft voice. I suppose an exception would be someone who is visiting or converting, but such a person is also capable of looking up the information and should, again, be able to ignore distractions for the most part. When I was converting I usually sat in the back, where I could both observe other people and the altar, as I hadn't the benefit of knowing other Catholics with whom to attend Mass, and sitting in the back didnt impede my seeing the Mass. I will admit that I was distracted by children at times, and even complained about them, but that was my problem and not the child's or his family's.
A child, however, is still learning the Mass. He also has less self-control and so is not as able to ignore distractions. Sitting in the back would make it impossible for the child to see, thus making it more difficult to learn the Mass and increasing distractions for the child. While one person doesn't trump another, I think being mindful of the impact these things can have is important. Surely it is important to give the children the best start possible in their faith by accommodating them in this way. Obviously I don't mean one should give in to their every whim and let them run wild, but accept that, in teaching them the proper behaviour, there will be blips and distractions.
I suppose part of this is my parenting philosophy, too. I've no doubt I could get the kids to all be silent (or at least nearly so) and still(er) if I allowed toys and snacks, but I don't. The only books I allow are prayer books or books about Mass or Mary or the like, though it is rare that we take even those to Mass. I want them to learn that this is a special time and place, and that playing or looking at other books isn't really what we should do in Mass. Snacks would end up all over the place, and besides, our pastor has asked that we not have food in the sanctuary, so unless it is absolutely necessary I will not do that. I also try not to be overly harsh, so i do allow some leeway, in that they can move some in the pew, but not get out of the pew without permission, for example.
I know sitting in the back wouldn't ruin them, of course, but it does make it more difficult in my experience (I used to sit in the back until told by my confessor to move for the sake of the children). I can already see fruit in this approach; it may not be for everyone, but for my family it is the best option. It certainly makes it easier on the children.