This is a personal question that you yourself must answer. Were you comfortable or uncomfortable that he touched you this way? If you were uncomfortable, did you tell him? Why or why not? If this were me, knowing what I know now about men, I wouldn't sit on a guy's lap. This really can arouse a man sexually and if that is my intent and he is okay with it, then that's fine. Otherwise, you must judge the situation for yourself to determine your answer.
Me and my friends were sitting on a bench. I sat on my bestfriend lap. We Know each other for 10 yrs. he put his arm around me, and later he played with My shirt. (unbuttoning It on the back) I wasn't facing him. Is this okay?
Let's ask the person this should really matter to . . .
"How did it make you feel?"
If you can address your feelings to what happened and you are okay with it, you might be moving on to being MORE then friends . . Or in need of a physical connection . . . .
If YOU are not okay with this, then you need to communicate this.
Well, if you met in pre-K, even 10 years older is still too young.
All (98%) of us humans are hardwired to "mate." And every woman is a "potential" mate for every heterosexual male---and vice-versa.
That is the basic reality that underlines your question "Is this okay?"
A general answer is that it's normal, but not all normal things should be done unless other considerations of (unintended) consequences and a general understanding of how this kind of thing fits into your life at this age.
I certainly can't tell you that this is "okay"---even if you feel great about what happened or think what happened was great.
You need a lot more information about the stage of life you are entering and how to navigate it successfully.
Please start talking to an adult you trust to explore what is happening with you.
My first thoughts are if you have to ask if that is okay then obviously it is not okay with you. Don't sit on boys laps, friend or no friend! Have respect for yourself and they will to.