Manal,
This study shows a definite improvement in red blood cell health and survival of these cells by supplementing with magnesium and immediately draws a connection in my mind between this longer RBC life and the reduced gallstones that were found in people supplementing with magnesium. It does make sense that if the RBCs are healthier and live longer, that this would reduce the amount of bilirubin being produced by RBC destruction and hence, fewer gallstones. This is completely a surmisal on my part but it does seem to follow. So, as I have been saying since 2003, take magnesium with your calcium and don't forget that vitamin D is needed for absorption.
To get back to your question, yummi bears are a good multivitamin supplement but other vitamins and minerals may be needed to attain the amounts needed for thals. Magnesium is definitely one of these minerals. An example of a good supplement for a child his age can be seen at
http://www.puritan.com/pages/file.asp?xs=24D2AE024614456090AA40ECA50706F8&PID=4442&CID=1&CPID=6792&rlid=For his age, take one or two capsules daily.