Given that the stock is that old, and cost basis is not available, I'd assume all of the sales is capital gains.
However, if MIL is filing single and has income less than ~36K, then most of those capital gains are taxable at 0% (ie not really taxed).
Alternatively, she could gift them "in kind" ie as stocks and the grandchild sells them, then no taxes (ie no gift tax if<14K).
The amount is small enough to do either way, but the hassle factor will be the key thing.