Wood sealer for seat?

If you fall off any kind of swing, you could get hurt. Part of playing is taking risks, little ones, and finding what you can do...and finding out what you cannot or should not do.

Spar varnish stands up well to weather exposure. However, it is highly glossy and therefore slippery when wet. Me, I learned not to be too stupid with things that were slippery. I figured that out the hard way and am a more well-rounded adult as a direct result. Take that advice however you like.

The most enjoyable activities are often those with an element of risk, even risk of death. So don't worry about people falling off, many people do it deliberately. Don't expect any swing to last forever because you will just get disappointed when it fails. Kids test things to destruction. A fancy seat is not essential - a piece of stick, or even just a strong rope to swing on is a great toy.

I would use spar varnish. It is more flexible than other varnishes. It is designed for use on ships spars when can get very wet and expand and contract.