Ok so wife cam driving up with these two posts sticking out of her lil kia and said she wants them for a swing she's been wanting. Planning there 8' tall so figured I'd put them in the ground like 2-3' and quickcrete them in place but how would y'all do a cross beam for the swing to actually hang on ? Securing to the top that is.Was thinking 2 8" construction grade lag screws through too of the cross beam and down into the top of the poles but seem like they could shear off possibly. It's just going to be a little one person swing for her tk ready her books in under the pecans tree out front so it's only going be holding up like 400lbs at the absolute max. How would y'all go about it ? Still got a few days till I can put it up due to the rain and waiting the chain to get delivered

So, swing for one, for her, 400 lbs? I wouldn't let her see this thread, or you will be in deep do doo.

Well shes 150, swing probably gonna be like 100 when I'm done and the other 200 i just the safety net

Single swing, or a bench swing?

Those posts are too small. You need minimum 6"x6" square posts or even larger diameter round.

A very heavy duty cross member too. At least 6x6.

If you go with those 4" or 5" fence posts....make sure your homeowner's policy is paid up with a liability clause.

As sawmp wolf said, 6"x6" minimum. And you want to use the rule of thumb for cantilever, in reverse for this application: 1/3-2/3. In other words, at a minimum, if you have 6' above ground, you need AT LEAST 3' below ground. Definitely concrete them in. Don't know your soil, but I would use at least 4 80# sacks per post.
Depending on your span between posts, you could easily use the same 6"x6" material for a span well over 6', and for attachment to columns, Simpson post-to-beam connectors with recommended Tico nails and through bolts would be fine. Or, less gaudy, you could use long 12"x3/4" lag bolts through the beam and into the top of said posts. You will need to pilot the holes and will want a quality impact wrench to sink them.

Welp these seem kind a over kill honestly but if y'all say so ,they went much more then the poles so....now ....lifting the top one up