I made the kit kat cake last year as a combined birthday cake for DDs 1 and 2 and it was a huge hit with everyone.

Ones I've made for my kids that you haven't already done:

a cloud fairy - simple round cake that you then cover with small white meringues and stuff a fairy on the top.
A castle - the one I made was very pink but you could use different colours if you did a combined one for your kids - square cake with sponge rolls standing on their ends on each corner, ice cream cones on the top for turrets and then lollies used around the outside "walls" for drawbridges, flowers and vines etc.
A turtle
Caterpillar made of lots of cupcakes

Can't think of anything else for the moment but will come back if I do. Don't you just love birthdays close together!!