I think that if the kid obviously initiates sitting on your lap etc. and its obvious to the parents that he did that, then its ok to let him sit there etc.Unless he's between, say, 13 and 18. I think after that he can decide himself. At that age he prob wouldn't do that in public anyway.
I think its also important not to make a big deal about it. Act maturely as if its no big deal either way, and its okay to show a little affection esp if they show it first and esp. if you are in a Christian environment or whatever where wholesome affection is understood and not suspected.
All depends on how well the parent/s know you too.
I think its wrong to stop showing affection to your own kids just because society is paranoid.
If my 12 yo boy (if I had one) was to jump into my bed at night I'd have no probs letting him sleep there. I'm his dad, after all.
We need to stop being so self-conscious and scared of affection--while remaining aware of people's fears when dealing with other's kids of course.
A big factor as I said is how well you are known. If you are known as a "normal" person with normal interests, a family, friends, pref a girlfriend! and act fairly normally and people are familiar with you, and kids obviously like being with you, parents should tend to be ok with you being close to their kids, if you don't overdo it.
A quick squeeze (bearhug) or playfight or playpunch or hair mess-up is often acceptable.Especially if done in a playful, slightly rough way as boys seems to like, I've noticed.
Its good to learn how to do these things spontaneously with ease and confidence, with that down to earth decent fella big brother sorta presence. It requires being happy with yourself and confident in your own space and body. Imagine you are holding a beer in your hand amongst the boys at the pub, if it helps! Different energy to tender or sexual. Tender is good but can be more ambiguous, it takes more familiarity and/or sublte e knowledge of boundaries and self-confidence. getting all anxious and intense about a tender caress makes it look sus.

Happy holding!

Bertil.