If you're not willing to pay, which suggests it's not that big a deal to you, the easiest thing to do is to get to the first airport as soon as possible and then sort it out there. Formally you can call in 5 days beforehand, but given the complex ticketing I can see it wouldn't work very well.

I vividly remember a HEL to LHR flight when I found myself sat next to a 12 year old, parents left him there and went right down the back, and were also separated. Didn't seem to bother the family in the slightest. The CSD - on his final checks - asked me to tell my child to remove his headphones - those were the days. I said that I would, but he wasn't my kid. At which point, and we were trundling down Vaanta's main runway and about to take off, all hell broke loose, if you will pardon the pun. So I'm quite sure that at the airport they will shift other passengers around to ensure that each child is next to one adult as a minimum.