Building a raised garden bed - first time gardener

The plan:

• 4x8 ft cedar bed
• Start with tomatoes, beans, lettuce
• Kids can help fill soil, plant seeds, water

Obviously I'll do the actual construction but I want them to own the planting/maintenance part. Any tips for keeping a 6yo engaged with a garden when nothing grows overnight? Also, where do people actually buy good soil in bulk that isn't stupidly expensive?