I have not seen any DMSA tablets. VRP makes 25 mg DMSA capsules.
All other makers seem to make only 100 mg or 250 mg capsules. I suggest that you start with 12.5 mg per dose. Opening a capsule and divididing the contents in half isn't that difficult. I have found that opening capsules and dividing the contents is easier and seems to divide the dose more evenly than cutting tablets in half. Of course this won't be exact, but good enough if you pay attention while doing this. You will need some empty capsules. Many vitamin stores sell empty capsules.

While dividing the contents of a capsule in half isn't that hard, dividing it in 4 even parts is much more difficult.
If you buy the 25 mg capsules, it should be easy. Trying to get 12.5 mg doses from 100 mg or 250 mg capsules would be quite a chore.