This is strange ... Is this the case for all children on the tree/a sudden change in display, or just the case for one specific couple and their children?
If it is only a handful of people, maybe try deleting them and adding them back via "Add child"? And if it is a persistent issue, contact support?
The problem should not be (only) connected to tree size, as my biggest trees are far bigger and I never encountered a comparable issue (though there are no pets on my bigger trees).
