For what it's worth I think this is potentially a huge safety issue. I am a professional pilot on the B737. Yet I know that, if I were a passenger involved in an evacuation I would not allow myself to become separated from my young children. If they were not sat with me and an evacuation began, I would move heaven and earth to get to where they were before I worried about getting out and, to be honest, I wouldn't be able to care too much about other passengers in the process. The only way I can explain this is to say that, absolutely, being a parent comes first. What is more, I doubt any parent would be different and I don't believe they should be obliged to forget their kids and get out with the flow.
Incidentally it's not just the low cost carriers who can't be bothered to accomodate young children next to their parents. I remember checking in to a BA flight recently - in front of me was a father (who had checked in on line, with seats allocated, the night before) being told that, due to some administrative cock up, he, his wife and four children under six years old would all have to sit separately. I think the youngest child (just over 2) got to sit directly in front of her mother. Other than that they were all quite literally rows apart from each other. Being the home of wonderful British customer service, the cabin crew did absolutely nothing to help, didn't try to move passengers about and then shouted at the 2 year old so much she spent the whole flight crying. Nice work BA. And very very unsafe to boot...