I think BM's suggestion near the top of the thread is a good one if you're going to do it.
It should be a bonus, a gesture, not an obligation. Tradesmen can be a suspicious lot. Too much attention can sometimes be taken as "spying on progress."
These days, they, particularly sub-contractors, tend to raid the local shop before they get to you. The junk-fodder they load up with is unbelievable.
One on arrival; one around 10:30 for "Popmaster" on R2; maybe lunchtime; then maybe afternoon tea. It's always well received and appreciated.
If it's only one or two guys, then biccies if you like. Any more than that and you'll need bulk-buying.
As Tony said; be careful if they're on daywork (so much an hour). That could be a lot of downtime at your expense. Although, once someone is into their work, they often drink their tea "on-the-fly".
You sound like a good customer to have Jo ;o)