Theoretically adding the lap child at or near the time of reservation should make it less likely for you to lose your seats for invalid assignment. In practice, this doesn't seem to work. For instance, I've flown with a lap child only to find myself seated next to someone else with a lap child, which is not allowed since there's only one extra oxygen mask per row. You'd think the computers could have noticed this and prevented it, but instead the FAs had to force somebody else to switch with my neighbor.