Okay responding to all of the above:
I am interested in plecos in their own right... smaller ones than the "common" pleco. I feed the bristlenose I have now plenty of veggies. My daughter loves to watch it suck on the glass and the way it swims so I figured she would be interested in one. I don't keep any animal just as an addition to other animals (for instance I don't keep things just as an "algae eater". I make sure that anything I have is getting it's own diet so it is as healthy as possible. Which includes snails... even the little pest ones.
We already have some smaller tanks in the house, and she has been helping me clean the 20 gallon and the betta's tank. She "points the hose" for the 90 into the bucket and finds that a very fun job, and she helps with various other smaller tasks. I understand a 120 will require "help from Mom" and am okay with this, I want it as something we can do together. Also, kids like a large variety of fish so in a larger tank she won't crowd them, and I think she will get more educational experiences out of it (she can actually see schooling and breeding and such). I also think it's harder to make mistakes water-quality wise with a larger tank, which may be helpful for a kid. Really, I'd consider all the tanks in the house "our" tanks, and this will just be the one located in her room. There is also the fact that when she is older, she'll already have a great tank to start off with if she chooses to stay in the hobby, and if not I'll have a great tank to keep. Judging from how much she loves fish and how much she has already learned about them (she DEVOURS knowledge of fish), I'd say that she is a die-hard aquarist already.
I also like that a taller tank is harder for her (and friends) to "play in", she will need to ask assistance until she is old and tall enough to know better and really understand what her fish need. She seems very disciplined in this area, though. She didn't take after her mom in that she doesn't "get into" things if she is told she is not allowed. There is also the fact you can pretty much tell what anyone is doing in our house, as it is not very large.
So amano shrimp and ghost shrimp won't eat the small fish in the tank?
Thanks for the other species info and suggestions, if anyone has any more I am interested.
I think at this age she is more into trying lots of different things and would probably get bored with a species only tank (oscars for example). Also she is soft-hearted and I doubt she'd be into feeding a fish other fish.
Currently one of her favorite things to do is sit in front of the 90 gallon and make up stories about what all the fish in the tank are doing and name them all (the names change frequently). She can do this for 2 hours without seeming bored. She likes to pretend they are going certain places or make up conversations they are having. This is another reason I think a variety would make her happy.