Hardik,
It takes time for S. ferritin to come down. It took more than two years to see a significant downward shifton our child when she first started chelation. The best way to know if the chelator is working or not is liver MRI.
for the stomach ache, try giving Zantac (Ranitidine) to your child. My daughter has significant stomach issues from using Exjade (even after taking Exjade with food, right after food, split dose and so on). so now she is on Zantac 75mg (twice a day - morning & evening) and the stomach issue is under control. Also we have seen that if she is eating every few hours (and drinking enough liquid) without a long gap between meals, the stomach issues are under control.
Your child is only 2.8 yrs age? if that is the case, In my opinion, don't feed him/her too many supplements. Just healthy meal at regular interval, lots of fluid, dose of ranitidine (to keep stomach ache under control), one multi-vitamin (iron free) and the chelation med. That will be enough.
I will suggest not to do any other chelation without doctor's advice. Just keep up the chelation with the right dose and get the side effects on Desirox under control. it will take time but it will work.