Amy,
I think that a Newf should be just fine for your daughter. The caveat to that would be to insure that the Newf you handle has basic obeidience training down pat. Ideally, you want the Newf to be responsive to leash training, so that there is no pulling issues.
My 9 year old walks Onyx, and we are looking into Pet Therapy to join into. Onyx is very well behaved on a leash, and we're working on using touches to either flank as commands, instead of voice. (Very similar to horses actually) This way, in case my son can't makke himself heard, Onyx will still know what he's supposed to be doing.
I guess where I'm going with all of this is, if you and your Daughter want to use a Newf for 4H, go for it.