It's really not that uncommon. A 32 year old with a 13 year old child would mean 19 when the baby was born... I'm 31 with an 11 and 9 year old...
Now, to the OP - if you already have an artist picked to do the tattoo, the best idea would be to talk to your artist. If it's someone who has worked on you before and is confident that you will be coming back to him for the work, give him the list of ideas like you gave us, brainstorm with the artist and have him draw up a custom piece for you. You can discuss size and placement with him as well, and will end up with something perfectly sized and designed to fit the area of your body...