I think you need to set the expectations upfront that you allow upto two child for the free version and allowing upgrade if they have more than 3 child to avoid after shock.
As rightly, pointed the problem with CHILD 1, Plus CHILD 2 Approach by Danielle there are chances they might forget which details they entered i.e Younger or Elder.
Note: You might give an option to Add addiotional child if a parents want to and in that case you need to pull in the upgrade version then.
