Hello and welcome to Tradgang!
Ddepending on your local ordinances, you may be able to set up a target in your yard. If not, a lot of guys practice in their garage or basement. Either would have plenty of distance for a 9 and 5 year old to learn and have fun. A basement under a ranch style house can sometimes provide pretty good shooting. I expect though that many homes out your way do not even have them and are just built on a slab. If your yard is fenced, you might be able to have someone build you a backstop against the side of the house that is big enough to catch any stray arrows and shoot towards your house. Shooting in other directions sometimes makes neighbors nervous.
One thing for sure, realize that bows are NOT toys. Even a kids bow with target arrows can cause serious injury or even death. NEVER allow your kids to just take them out on their own and shoot unsupervised. Always stress safety and if two of them are shooting, make sure that they are not standing too close together and that they are both the same distance from the target. Never allow one to shoot from say 10 yards and the other from 15. Kids are kids and accidents can happen. Treat bows like guns. NEVER let them point their bows at each other or anyone else even in "fun" with no arrow. A good rule to follow is that unless they are both ready and standing at the shooting line, they do not even put an arrow on the string. If they see you are serious about them following the rules, you should have no problems. Kids love shooting bows so much that it's pretty easy to say, "well, if you don't want to do it the right way, you can't shoot at all." They almost always agree to do it the right way! Might even be able to use their archery time as an incentive to behave...
On the plus side, archery is a great way to instill basic discipline and responsibility. Now, you need to go out and get yourself a bow and join in the fun!