About how many grams of food should they eat a day?

I don't think there is a correct answer to this question. Obesity is usually only a problem if the diet is too high in fat. Even for adult birds, level of activity, molting, and other factors can influence how much food a bird needs.

Mine eat more some days and less some days, and I've only ever had an obesity problem with one or two cockatiels in a community cage that ate too much seed mix. (I gave the tiels seed mix if they were outside in the winter, they needed the extra fat then.)

Weigh the bird periodically and handle it enough to know how the chest should feel at the proper weight. Then, if it starts to lose or gain weight, you will feel it in the chest and know to go to the vet or correct the diet. An obese bird has a "soft" chest that protrudes out on either side of the breastbone, and also often has a bulgy fat deposit just in front of the vent. Birds that I have acquired that needed to lose weight, I didn't feed them less, I just fed them different things - more vegetables and fruits, no nuts or seeds.

However, I have NEVER restricted pellets on any bird. They lost the weight anyway. I think they are sort of like people in this respect: I will eat too much ice cream, cookies, etc. and gain weight, but I don't eat too much salad, fresh fruit, etc., even if it is available in unlimited quantities.

I'm sure there is some bird out there that exists only to prove me wrong, but I've never heard of a bird that overate and became overweight on pellets and fruits & veggies - it's always the seeds, nuts, and high-fat people food that gets them.

(Like PIZZA, you silly Quakers! You'll all =need= to swim or do something for exercise after eating that high-fat pizza!)