Personally, I do not think kids are ready to start learning a basic system until they are at least 10 years old. Any younger and we should just try to keep things about having fun. If you are coaching kids that are 9 years old or younger and you can get them to fall in love with the game, learn a proper jump stop and a proper defensive stance, you have done wonders and will make the next coaches job a million times easier.
For my 10 & 11 year olds that I work with, I have a series of basic drills that teach proper cuts, how to set a screen, jump stopping, and all the other basics of perimeter offensive play. What makes my system of drills effective is that EVERY single drill is actually a small piece of our offense that I will be installing later. What I've done is take our offensive system and break it down into as many small parts as possible to form a series of simple drills. Each drill has the players specifically on the spot of the floor where they will be once they are running the more complex system.
Over the course of about 3-4 weeks, the kids should have all of the smaller drills down solid and they will naturally begin to get bored with them. And that is when you start to piece it all together. Instantly once they start to run your offense as a whole, they will recognize the relation to the drills they already have well versed. The transition will be extremely quick at this point and more importantly, it will be fun and engaging for the kids and will instantly re-energize their enthusiasm for practicing. The more small stages you can break down your drills and development, the better off you will be. It will be easier to teach, and on the flip side, the kids will feel like they are accomplishing more as the drills get more sophisticated.
Once they are able to run your offense as a unit, then they are ready to do it with a live "dummy" defense. There will be a slight regression and adjustment period when you do this. This should take about a week or two of practice before they are comfortable with all of their cuts & screens with the defense on the court.
Once they then get comfortable with that, they are then finally ready for what they've probably been begging for since day one, which is a live scrimmage. Make sure it is a controlled scrimmage. Stop play if they are not sticking to the offense. Make sure they understand how to reset a play/offensive possesion. Keep things slow and make sure they are boxing out and understand their roles on inbounds and outlet passes.
The key is to be patient and take everything in logical steps. Rome wasn't built in a day and neither will your team. They will not nor should not be expected to fully understand and run your offense the first half of the season in games. If you can have them fully up to speed by the mid to 3/4 point in the season, you have done a great job as a coach. And never forget to revisit all of the small drills you did in the beginning so you can re-enforce good habits and get even more specific in your critique and evaluation of their growth.