First, I'm going to suggest forgetting about the muzzle brake. They are effective in reducing recoil but are hard on the ears, especially if shooting i an area where sound can be reflected back to the shooter - an enclosed stand, trees, steep hillsides, beside big boulders, etc. I cut loose with my .357 one day while standing on a 45 degree slope. Hearing in the ear on the uphill side took over a day to return to normal and even then I think there was permanent loss. Let your kids shoot firearms they can handle and if they can't handle them, let them grow a bit.
Never been a fan of the .243 Win for large deer, preferring the heavier bullets available in the .257 Roberts. Generally I recommend a .30-30 over a .243 Win as it is more than adequate for ranges most kids should be shooting. Beyond that, anything up to a .30-06, with reduced loads if necessary.