Michaels.com might be the only thing I need to say here. But it definitely isn't!
Tip: If you have one near you, I strongly advise you to get to one and check out their kid section. But be warned: you WILL spend money! They have tons of great ideas for any kind of craft you can think of. But tell your boss first. Explain you'd like to get some crafting supplies and go in with a budget.
Sometimes they have dollar isles and you can find anything from stickers to markers to coloring books to puzzles and games. They also have a great selection of canvas items like bags, puppets, costumes, and decorations. Canvas is a wonderful material because it's durable and easy to color on.
Idea!: Kids will LOVE to decorate their very own tote bag which you can fill with chalk, bubbles, and water toys...they'll have a blast carrying it to the park on your next outing!