If they helped you plant some seeds in the spring and then have been at camp all summer, make sure you take pictures of the flowers or vegetables later on, have them printed on disk or use a scanner and send them off for them to see what happened after they left. If you made preserves or jelly from the vegetables or fruit, be sure to save a jar for their next visit or add it to the next package you send to them. If it's at holiday or birthday time, enclose a book on activities in the garden and bookmark a couple of ideas you want to try next year with them. Check the TV guide to find out when particularly interesting nature shows that might appeal to a youngster and make a copy to enjoy together when they visit or to send along to them as a treat. Tapes don't cost much and can be re-used.
Kids love to get mail and packages are a really big treat.
If you have your own video camera send along a tape of you in your garden, perhaps standing in the vegetable bed they helped you plant. They'll treasure it always. You can draw up your garden and scan in the plot to email them. Ask for their advise on where to put things next year, what to grow, what seeds to order. Whatever gardening activity you choose, remember to keep it simple and fun.