Hello D.!
From the description you provided of the age span being 9 years, I presume you are trying to figure out party plans for the 3 year old and the 12 year old.
Probably one of the easiest types of parties for really young children is the Pizza Party type. In one of those places like Chuck E. Cheese (I don't know if you have one in your city) but these types of Places are kid havens. They have built in entertainment, the whole place is an adventure and they serve you food. The only work for Moms is escorting the kids to the age appropriate play areas.
You could then expend more energy (and more money) on the party for your 12 year old. At twelve kids are still into themes. I hear there is some kind of Hannah Montana sensation, maybe that could serve as your theme. Invite all the guest to either show up as Hannah Montana or have a Hannah Show as the central theme of your celebration.
If you want a more mixed gender party, plan some kind of Treasures of the lost temple type of theme, wherein all guests are given some type of clue (perhaps in their individual invitations) wherein each invited guest will have to "discover" their hidden treasure during the course of the party. Of course all successful treasure hunters turn in their find for a real gift by the end of the party.
There is also the incredible fun Party and Sleepover. This is a never ending party of dancing, food and fun. The girls might give each other a makeover (provide them with inexpensive wigs, make-up, faux nails, washable tattoos and clothes-perhaps from a thrift store). The girls can turn themselves into "stars" put together performance routines and lip-sync a performance. It ought to be hilarious. They end the night in their favorite pajamas, wake up to a splendid breakfast and their moms pick them up before 10:00 A.M.
Well those are my ideas!Have a great time.
Sincerely,
Angie