While some kids will take it in their stride, gyms need to be very careful about the way they increase a kids hours. Increasing days is also important to do slowly. A gymnasts should really only ever add one day at a time to her schedule. It is not advisable to jump from 3 days a week to 5, without doing some time at 4 days a week for example. It is preferable to add once day at a time and wait 6 months before adding another. Gymnasts who previously loved it can burn out when extra days are added too fast.
A gymnast who trains 2 days a week still has most days to herself while one who trains 4 days a week is now spending more than half her days at the gym. Other activities can go by the wayside like homework time, family time, TV, time, play with friends, other sports etc. If the gymnasts feels they are missing out on too much else they will burn out. When days are added gradually they don't tend to notice that they have had to make some big lifestyle changes. If your gymnast seems less than keen to go to training at times and asks to skip days here and there she is probably doing to much. But if she is always wishing to do more then she is probably doing just the right amount. Increasing days and hours does not necessarily produce better results. A gymnast should be using the time they do have to the full before increasing hours should even be considered. A team doing 9 hours a week and using those hours well will easily progress much faster than a team doing 16 hours and wasting a lot of time (this of course comes down to the coaches too).
Parental burn out is another thing to watch out for, gymnastics is a big part of the parents and families lives too. When the kids do extra days the parents are spending a lot of time driving to and from practice and fitting dinner and homework around training. Other kids in the family are getting less time and the family can tend to feel that gymnastics is taking over their lives. So even if the child seems like they will cope easily the family must be considered too. It can be a shock to the system for a parent to go from driving to gym 2 days a week to 4. When its gradually increased they tend to notice less as well. And we want to keep the family happy because without them the child can't do gym.
Hours too should be increased slowly and practice should really be increased at 1/2 hour increments, generally 30 mins per 6 months. Kids bodies take a while to adjust to the increased training hours as do their minds. Towards the end of the practice they will get tired more easily and this can lead to accidents. If a child is tired they are far more likely to be seriously injured in class. Also it leads to progress going backwards not forwards. When a child is practicing while tired they are going to lose some technique, they are then practicing poor technique and getting very good and doing things badly.
Longer training will also mean less home time for the activities of the daily grind like homework, dinner, baths, sleep and so on. We know organization is the key to this but if done to quickly kids and families burn out.