Luang por isn't wrong by the sounds of it, I believe it is over 75 they get called it, but I am not sure of propper Thai protocol here, but I also think it is for anyone who is of a certain age and a chao Kung, or preceptor??
but it is always better to be safe than sorry
and being overly polite is better than being rude through caution in this case I think.
I am sure any Dana you give would be welcome, why not invite them to ask for something specific that they need?
just be careful with fruit juice and time it is offered, some may not accept it for the afternoon, and try to get it smooth if you want to offer it for the afternoon, but a local maybe able to help better for the needs of the monks than anyone here!
Have a nice time you lucky lucky b******, what I wouldn't give for the romans....
wait, watching to much monty python