Hello, Christina!
You've picked a wonderful restaurant, and an even wiser way to use your Table-Service entitlement...the Fantasmic! Dining Package!
I'll advise you based on the assumption that you are on the "regular" Dining Plan, which entitles Guests to one Table-Service meal (one entree, dessert and non-alcoholic beverage), one Quick-Service meal (one entree, dessert and non-alcoholic beverage) and one snack per person per evening of your stay. Notice that appetizers are not included in this plan. That being said, you can order
any of the entrees on this
wonderful menu, from the Margherita Flatbread ($11.99 at the time of this writing) to the Charred Strip Steak ($28.99 at the time of this writing).
Now, because you are using your Disney Dining Plan Table-Service entitlement to book the
Fantasmic! Dining Package, you are each
also entitled to an appetizer at Mama Melrose's Ristorante Italiano! Take another look at that menu now, to see what a really fabulous decision you made by booking this Dining Package using your DDP! Yum!
Regarding the spaghetti cupcake, by all means, ask your server when you are seated if this is a possibility. We have often asked for a children's menu item even though our children are now old enough to be on the adult plan (anyone over the age of 9) and have never had a problem doing so. The reverse, of course, is not true. I would imagine that if it is at all possible for your children to have this great dessert (meaning as long as they haven't run out) then I'm sure your children will get that cupcake for dessert!
Have a wonderful trip to WDW! Be sure to come back with more questions as you think of them! We love to help!