well, some kids have no choice
Bak Siulam judo etc. are perfectly fine, so is Hung Kyun. the emphasis is of course not on hard drilling, but learning and fun. short sets like
mui fa kyun,
Lau ga kyun or
bang bou that i usually do not teach to adults are cool for kids. after one or two of these sets i would introduce some weapon - monkey staff or sabre for boys, straight sword or spear for the girls - more fun, plus a sparrign set like
fu hok deui chaak. kids have to learn, learn and learn, does not matter that the form is not perfect. additional exercises like little bit of running, swimming, tree climbing, stones throwing, climbing, easy stretching (hips are no problem, just be careful with the spine), any type of pulling, pushing, fun wrestling, breakfalls etc. are cool.
my sifu has couple of kids students, ranging form five to ten, and they are doing just great, even sparring sets. fast, strong, focused and still having good fun