I think you should just be straight up, why not start small though and use the dog as an example? It might sounds silly but she is part of your family but she didnt come out of your tummy
DD was just chatting to DH and discussing who came from whose tummy. I hadn't told him about your suggestion. DH then asks DD 'where did Rosie (the dog) come from?' DD thinks, and says "nanny's tummy!",