You're going to make a gorgeous cake for this littlun :-).....
Marzipan, homemade will be fine, as it's just ground almonds and lemon juice and egg white and sugar and vanilla essence. The pure vanilla extract is the best one, about �3 for the bottle but will last an age, if your making the marzipan, if not check the ingredients on the pack.....
The Doves farm is plain, so 1 tsp of baking powder to 8 oz of flour is the norm. This flour needs a bit more liquid than you would usually use, as it comes out drier.... if you have time a little practice one may be in order....
I haven't used dairy free chocolate, but if you look at the back of the flour pack, i think there should be a chocolate brownie type recipe on there. I have used that recipe and it's yummy.....
I guess rule of thumb is that you make everything from scratch, including the icing if you can.
The jam I use is St Dalfour, again from the usual shops...
I wish you the best of fun making your lovely cakes, what a lovely gift :-)...
B. xx