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.
I was brought up to believe the use of "going to go" was very poor English, we were encouraged to just say "going" or "planning to go". I believe DL originates from the USA, is "going to go" common in that country?