The only way to know if your kids aren't getting enough from you is if when they are weighed, they are losing weight, not gaining.
I stared cereal (mixed with formula) when my daughter was 5 months. I had stopped b/fing by then. A lot of people will come on here and say your child really doesn't need cereal at all, especially if you breastfeeding, but really it's to help give them some variety and get them ready to experience new textures (practice for real food). I think it's a positive thing.
If you are worried about your milk production since you are feeding TWINS (oh my god!!), i would call a lactation consultant. But just make sure she's flexible and not going to push you into anything you don't want to do. If you really want to give cereal a try, than do it! Just wait until they are at least 4 months old. You only give it once per day anyhow...there's nothing wrong with experimenting for now. also, I'd talk to your pediatrician as well. Just an FYI, don't give the cereal in a bottle.
Good luck!