Translation:Tomorrow, we are going to go to the beach.
Actually, the french sentence is quite disturbing because of "allons aller". In fact, using the verb "aller" ("to go") as an auxiliary is a familiar way to express the future, but it can be confusing. The strictly good way to express the future is to conjugate the verb at the future (in this case, "demain, nous irons à la plage" )
And you will have the reward by translating as "tomorrow, we will go to the beach", which is the exact translation of this familiar form.