Interestingly, none have asked how they got in there yet! (my eldest is 8)
Hehehe my 3yo asked me the other day - you have got off lightly getting to 8 without being asked
I was caught off guard (we were driving to the swimming pool of all places!) so I just said 'well I grew you there yeah?' and she was happy with that. Next time I will think a bit quicker and come up with something more accurate!

At the moment she is just interested in her being in there and how that worked, not how she got out so I haven't crossed that bridge, but I think being honest but concise and age appropriate is the way to go.
Funny story, she knows that there was a tube going to her belly button and that is how she got food, but the other day she asks 'where were my teeth to eat the food?' while gesturing to her mid section. Oh the mental picture she must have had!