So you're giving supplemental calcium? That's good, but be careful with the D. D is fat soluble and can reach toxic levels. Human D supplements are WAY to strong as is and need to be SUBSTANTIALLY diluted.
Their diet should be formed on the foundation of rodent blocks. Please see the following link.
http://www.henryspets.com/pages/Healthy-Diet.html
Henry's makes the only ones specifically for squirrels and if they each get two a day, MBD should never be an issue. Do they get blocks?
http://www.henryspets.com/squirrel-diet/