Grand kids need a swing and grandpa needs a game pole. I'm wondering if I lag screw a 4x4 between two 12" soft maple trees will it kill the trees?
Anybody have any exp. with this?
Thanks

Probably not, but I'd suggest tying the 4x4 to the trees with some stout rope, or lag it and rope it.  By themselves, lag screws are likely to pull out, particularly if the 4x4 is used as the main beam for a swing set.

Speaking from a carpenters standpoint, I would not trust anything without direct weight bearing.  Lag screwing to the side of a tree and applying downward force is asking for a system failure, especially with the constant motion created by the swinging of the kids.  A thru bolt system would be better, but as the tree ages, it expands which exerts pressure against the 4x4 and either grows around the 4x4 (which looks nasty), or it will exert enough pressure to pop the bolt head off or the nut.  In any of the cases above, I would strongly discourage the idea.

Instead, build a larger version of the bench swing that you see made from debarked logs.  You deffinately want some form of direct weight bearing coupled with lateral sway control.  Hopefully I was able to help with your question.

Much like the pictures below, just make everything taller to accomidate a gambrel hook.  All of the hardware (metal brackets) can be found at Home Depot.  All the supporting lumber is 4x4, and the top beam is 4x6.

 

 

Ditto mike.
He took the words from my keyboard.

My grandad chained a 8" pole across 2 trees to work on his truck about 50 years ago. The pole was there until he had cut them down about 10 years ago because of storm damage. If I remember correct they were Walnut trees.

Ditto Mike V's post. We've had two game pole breakings at the Shrewhaven camp over the years. One almost caused some cracked skulls (of course hanging 8 plus deer makes a difference). We now hang them from the front porch overhang the was built to support the deer and shade them at the same time.  I'd avoid the tree's based on our camp experience.

Mike is right on the money! The lag screws are an accident waiting to happen and grand kids are not replaceable.

Thanks Guys! I will take your advise.