I used to have a dirt cake every year. We always put it in a plastic flower pot and I did layers of crushed oreos and ice cream. Let the ice cream soften so it is easier to work with. You can also use pudding instead of ice cream. Then add worms throughout (they will be frozen if use ice cream). Once I had all the layers done it went in the freezer and was taken out about 10 minutes before scooping. I also put fake flowers in the dirt on top, just be careful if use candles too, we caught the flowers on fire one year. They also make candy rocks that can be great to put on top.
If you want plants instead of seeds, you can always plant the seeds a few weeks ahead of time so you have small seedlings. Also if you go the plant route, check Home Depot or Lowe's, a lot of time they have a sale rack in the back and you can get 6 packs of plants for 25 or 50 cents and they usually just need a little water and are good to go. I think they seeds would be great. Small sunflowers, pansies, or marigolds are a few I can think of. Maybe pick a few kinds and they can even mix and match a little.
It sounds like a lot of fun and what a great idea!