Cancer treatment alters a child's routine drastically. When kids are dealing with cancer, they are immediately put on a truck load of steroids. Those steroids will make them initially put on some weight, and because of the weakness - they cannot engage in a lot of exercise. The steroids do make them out on weight, and at the same time, make their leg muscles weak.
Sit-up’s or a small walk, an ideal 10 minutes of exercise is what should be mandated. 10 sit-up’s in the morning and 10 in the evening would be perfect for them. Straining them too much isn’t a good idea. Children don’t need that much of exercise. There have been some cases a Paediatric Oncologist mentioned to me, where children wanted to get into gardening. And she realised how big a risk that would be. To dig the soil, put the hands among the flowers and the pollen. That would be too much for a child to handle, with all the infection they are now prone to. So she advised on no gardening, they can water the plants instead which in its own way can be therapeutic as well.
No hiking, swimming in wild water or river is a big no too. In a swimming pool it’s ok if it is heavily chlorinated. A heavily chlorinated pool is advised because that would kill the germs. Also make sure that the pool is not too crowded. Ensure your child avoids drinking water from the pool (although very difficult to monitor for the little ones). Exercise for just about 10-15 minutes per day is what needs to be done. Keep a check on what you make them do, keep your doctor in the loop at all times. And no strenuous activities, just some very simple stuff to keep them occupied.