The first thing that you need to do, is to think out the blue prints of a whole routine that you could get the kids to perform. You need to go ahead and do a little analysis of all the items that are available to you, and ones that you can get to improve the routine that you are going to be using. Remember that these are children so they don't need to use any weights, instead, they just need to work on their stamina and they need to use their entire body.
This is why you should look to just do things with them that might be fun, while they help them get fit.
Now what you need to do, after you do an analysis of everything that you have in your backyard, is to actually plan out the whole routine. You can opt to lay out everything in the form of a track, and then suggest that whoever does the lap faster wins. Children love winning at games, so using this against them could prove to be helpful.
Just remember that the routine is going to be up to you to decide, so there is no specific thing for them to do.
However, you should remember to incorporate a few basic things into your routine and plan. Remember to get them to do sit ups, push-ups, laps and pull ups if they are old enough.
This will go on and target all the areas that they might need to work on and will help them stay fit and ahead of the others who might be as old as them.