You are the only one who can decide.
The first Maccas party DS1 was invited to was for a 5th birthday.

I woudlnt be worried about them asking for it every time they see a fast food place. Even if they do just say no and keep driving/walking etc.

Serioulsy, DS1 knew what McDonalds was well before we ever went there. Advertising and merchandising are everywhere. whether one likes it or not, unless one lives in a cave, there is a chance one's child may be exposed to it, and then ask for it every time they see the fast food outlet anyway. Or they might just say look theres maccas ocasionally like my kids do. The first time we took them there, DS1 took 1 bite and said 'yuk dad, can I go home and have a sandwich?'