If they are more or less proper trick bikes, more important is not a brand, but proportions. I see so many youngsters provided with adult bikes which take away whole ability to make any simple trick.

Also, consider looking after smaller wheels than 20" or with thinner rims/tires according to weight.
The optimum size for frame top tube would be 17.5"...max 18". Cranks 165mm. But also, if legs are proportionally longer, then(only) You can afford little longer cranks for easier peddling.