I always went to the gym as a kid (my dad being a personal trainer at the time). The focus was always more on cardio and flexibility rather then anything weights based. There is nothing wrong with children attending a gym, provided that they are supervised. They shouldn't do anything involving weights with the purpose of body building - (ie, free weights are not a good idea). But most of the machines are ok on low weight settings, as part of an overall routine. Certainly nothing wrong with the cardio equipment (ie, treadmills, bikes, rower etc). Also depending on the gym, it isn't all about machines - many have classes and other types of fitness equipment (ie, rockwalls, gymnastic areas, etc). So long as the children are taught the correct exercises for their age, and how to use the specific equipment their program has been designed around - there shouldn't be any problems, and it is a great way to keep them active, keep yourself active and perhaps even kindle a love of physical activity that will last them through life.