Awh, designing the game with your kids is really cute! Since you wanted feedback, I will try my best to give some. As for the design, I think instead of the color changing automatically as you destroy enough enemies of a certain color, it could be a lot more interesting if we got to change OUR color on the go to fit whatever enemy comes our way. I think this way because we need to destroy all colors of enemies anyway, so the bullets growing in size is merely a matter of time (rather than something we achieve strategically). Also color blind people may have a hard time differentiating between the colors, so giving each color of enemy a slightly different design could help with that.
Another point, while I should have expected the tank controls, it does feel a bit weird since it's not entirely clear where the player is going to go when pressing W at any given time (my eyes don't look at the player, they look at the enemies most of the time). And the top part rotating independently adds to that confusion. So maybe you could make it more visually clear where the player is facing? Perhaps even a simple arrow in that direction could do, or you could update the artwork to be more clear.
Anyways, great work, and best of luck to you!