Yes baby squirrels as well as adult squirrels can safely eat raisins and other fruits.
The feeding guidelines for squirrels are to avoid feeding squirrels dried fruit of any kind, figs, fruit juice, persimmons, plums, prunes, raisins etc.
But I've fed wild squirrels all of those foods and they've always came back for more so I doubt it will hurt them to eat the foods and if the foods were bad for the squirrels then they would likely not eat it anyway.
Squirrels also love to eat a lot of different vegetables and things such as apples, peas, watermelon, grapes, broccoli and yellow squash.
Notice grapes are okay for the squirrels which is what raisins are but just dried so I would not go by the guideline unless you want too.