for the initial tribal part i would go with a black fill and also a bit of red thrown in or even a blue.
as for the writing i would go with purple and green.

now i know these colours are different but each bring out the intensity of the other and yes it is random but if you watch any child of a young age. there choice of colour is almost random also. there not worried with the colour they choose. whether they mix and match or not. they choose the colour almost at random.

plus as you said you wanted it to be appealing to young children. a child isnt going to worry whether the colours go together or not. these colours do go well if matched properly anyway…. look at it in a childs perspective. bring out the inner child and give it a shot….

by the way i like the design and i think it will work out well. when are you thinking of getting it done?