Child support is always modifiable. The parties to a marital settlement agreement can preclude later changes by the court for most provisions. For example, they can agree on non-modifiable alimony, and then no court can modify it at any time for any reason in the future.
However, the same cannot be said for parties child support. No matter what the parties say in their agreement, child support is always modifiable by the court.
If your spouse agrees to no child support in return for transfer of the house, and you transfer the house, can child support later be claimed? Yes, child support is not the parent’s claim to waive. It belongs to the children. Similarly, an agreement that a certain child related-expense will not be covered, like orthodontic services, is not enforceable.