Reservations for children can regularly cause problems, there are a number of reasons, the most recent is caused by lower fares that do not offer a discount for children, in other words a family of 4 all pay the same price, adult or child, the system then just see's four seats and not necessarily that there are children involved.Staff at airport will normally do their best to change seat assignment , however if the request is made near to flight closure it is often easier to sort out onboard, as any last minute seat changes can cause confusion and duplication at the boarding stage, which is precisely the time you want things to go smoothly.