Are horse chestnuts safe food for squirrels?

Yes, grey squirrels like to break off and discard the leaves and stem ends of these particular trees in the spring. They must be eating the tender young insides - I don't know exactly what they are looking for, but you will often find piles of dead/dying leaves and twigs under the tree. So - yes, a horse chestnut tree will attract squirrels, but you won't like it!