Done quite a bit of searching with no results.

I want to fly 4 segments, A-B-B-A, with an infant in lap only on segments 1&4 (child care is in B). If they ticket the infant in lap on segment 1, and the infant does not fly on segment 2, will the invalidate the return adult tickets? Is it possible to then re-ticket the infant on segment 4 without any reprocussions?

Thanks in advance to the experts, solving the unsolved everyday.