My kids have been doing Tae Kwon Do for 2 years and just wondering if there was a better system for them for now. I feel they will become black belts within a year but not have mastered everything due to their age.
not have mastered everything due to their age
Age has nothing to do with mastery - mastery is a combination of physical refinement and academic knowledge. So you can do a kick perfectly, but that doesn't mean in itself that you've mastered it - you also have to know how and when to deliver that physically perfect kick.
I feel they will become black belts within a year
As already mentioned, a black belt is nothing, it is just another grade. Most schools also have a distinction between the kids black belt and an adult black belt - very seldom is the knowledge gained and the training undertaken anywhere near similar for each.
My kids have been doing Tae Kwon Do for 2 years
To get all Zen like, this training time is but a blink of the eye. Two years for most styles/arts will only take you halfway through the coloured belt grades. The learning and time taken for the later coloured belts usually gets longer and harder.
just wondering if there was a better system for them for now
Maybe yes, maybe no - we can't suggest what art would be best because we don't know your kids and have never seen them train. What really counts is the instructor they have, rather than the art they do. Cross training in another art is great and usually complements the skills gained in the main art they practice. This means they could gain from just about any other art they decide to train in. The only way you will know is to try them. They might even improve just by going to a different TKD school as a combination of a different teacher with different students may show up flaws in their technique.
My kids have been doing Tae Kwon Do for 2 years and just wondering if there was a better system for them for now.
At this age your kids should be having fun and developing gross motor skills. Taekwon-do might be great for them depending on the instructor. The instructor needs to instill discipline in the children, and at the same time teach them basic moves (and have fun learning, of course!). If your instructor is doing that now, then I see no need for a change.
I feel they will become black belts within a year but not have mastered everything due to their age.
It doesn't matter what belt they wear, a white belt who is eager to learn is far better than a black belt who doesn't want to train.
So don't worry! Your kids are too young to master everything in Taekwon-do. They should be learning the most basic things and build their knowledge/skills from there. Be patient and things will fall into place. Who knows, they might want to pick up another martial art or fighting style.