In reply to HeMa:
I was always happy with rigging a full body harness for my children. Just make sure the slings cannot slide under load.
After all, you are not normally trying to stop a lead fall! I was usually "belaying" them up stuff I had just soloed, and then lowered them again.
Once the children were a bit older we switched to proper seat harnesses, initially also with a shoulder loop rigged from a sling to prevent them from toppling backwards, as otherwise the harness would have to be pulled uncomfortably tight.
CB