You need to know if all parties are agreeable to how to allocate it.

If they are agreeable, they can allocate it using any percentage, but it usually has the best overall result to allocation 100% to the tax return that has the lowest Poverty Percentage on the 8962 (in most cases that is the kid's return).

If they don't agree, then it is allocated based on the number of "covered" people on the 1095-A.  For example if it has three people on the 1095-A (two parents and the kid), then the parents claim 67% and the kid claims 33%.