Had the same issue when flying Virgin Gatwick-Orlando-Gatwick. Children at the time was 3 and 7. We where placed on four different seats throughout the airplane. I guess the neighbour to the 3 year old wouldnt been too impressed to look after a 3 year old for 9 odd hours. Only after demanding to speak to the station manager at Orlando (Swedish lady, very helpfull), did we get seats together in the middle row.
On KLM when you check in online, you can not check in seats for the children. Recently checked in a party of 9 including one infant and one child. When all adults checked in, the system tells you that you can not check the child. Great! It was sorted on the airport but it just adds on the stress when flying with a large party and especially children.