Should that be "odwiedzimy", since it is about a one time occasion in the future?
I don't think so. "Odwiedzamy" is "we are visiting" in the present tense, but can be used in the future to, same as the word for "walking". The future tense for "odwiedzamy" is "Będziemy odwiedzał" or "Będziemy odwiedzać".
It is also correct. In questions your version would be brilliant ^^
Odwiedzimy = we will come and visit
Eg. Odwiedzimy was! = We will come and visit you!
Odwiedzamy = we are visiting (our grandparents regularly)
I use "odwiedzimy" more than "odwiedzamy". (Native here). It just sounds better.