Trees that size will not be harmer by screwing 1/2", 8" long eye bolts
into them. Predrill 1/2" pilot holes slightly angled downward and
coat the eye bolt threads with wax/parafin (latex works, too. Raw
latex is the natural "waxy" coating on leaves). 1/4" or 3/8" S hooks
to attach the swing's chains.

If you have some milkweed plants (Monarch butterfly caterpiller food =
latex), rub the leaves on the bolt threads, if you don't have a candle
handy.

Sonny