If you are going to do it, then decent ear defenders are a must.
I know you probably won’t know this till nearer the time, but if your baby keeps pulling them off their head then do not risk it. Your child has to be happy to keep the ear defenders on.
I certainly would not want to risk my kids taking them to an F1 event unless they understand that they have to keep their ear defenders on. Some kids are fine, others just hate hats and things on their heads.
The reason for GA tickets, would be that you have the option of sitting in open air and the noise will not resonant off the sides and back of the covered grandstands. I sat in a covered grandstand this year and the noise was louder than previous years of open seating and GA.
Just have a PlanB like someone to babysit if you change your mind. But then if you can arrange a babysitter, you might be better off with that. After 18 months you might just want that days break. :D