Upvote:4
Problem is clearly child 2 since he/she is paired with different parents on each leg. This may not be much of a problem if child 2 is under two and doesn't need his/her own ticket. How old ?
Option 1): Buy everything as one-ways.
Option 2): Call the airline and see if they can find a creative to ticket this. Perhaps you can book 3 round trips (parents + Child 1)and two one ways for child 2. Ask the airline to attach each one ways to the matching itinerary of parent for that leg. This may have a better chance if you have status with the airline.
EDIT: one more thought: I would recommend against having the kid on a separate itinerary. If anything goes wrong (delay, cancellation, re-booking) your are at the mercy of the airline to do the same changes in sync on two different itineraries. This can be very messy and there is a risk that you get separated which is totally unacceptable. Many airlines won't allow this anyway.