This is my go to chart. I always do party sizes and I've never been told that someone didn't have enough cake:
Do you use the serving guides? According to the below guide a 6" and 9" party servings would bring you close, but your tiers are so tall you should go with wedding sizes 8" and 6" or even 5".
don't buy a new pan -- cut 1/4 inch off all around the 10" cake -- you'll have a true 9" cake plus icing which is a little bigger than the serving charts allow for-- when you bake a layer cake you loose about a half inch -- a 10" cake'll come out about 9.5 inches -- and especially since they are so tall and the kids are so small (that rhymes :)
y'know if it was a party for a 13 yr old male baseball team -- you might wanna allow more but a (true) 9x6 that's 5" tall will be plenty --
it's not the idea to feed them a meal of cake it's a serving it's a party it's ok
9x6 in your 10" pan you got this -- that's 33 party size servings 49 wedding servings and there's a happy medium there somewhere -- don't bake too much anymore
Wow! Thanks for all of the awesome replies. K8memphis... I totally agree with what you're saying. Thanks for the help. :)