My honest advice would be to get them as 'family' pets. Me and my younger sister had our 'own' pets from a young age and i am NOT planning on doing the same with my LOs.
When me and my sister had Guineas at a young age, they were just manhandled all the time. The day we bought them, my 6 year old sister dropped hers on the floor (from her standing height) because it scratched her. My Nan ended up looking after them, doing all the dirty work, but that was because we weren't encouraged to actually look after them. (Thankfully, my love for Pigs remained and i obviously treat them very well now!)
I currently have three G'Pigs. My nearly three year old LO likes to look at them, help me feed them etc, she will stroke them occasionally and i teach her to be gentle, but i will continue to encourage that they are 'family' pets. I wouldn't allow her to hold them, they're far too flighty and i fear for them! She can't quite comprehend the idea that they are 'real'!
I think its a fantastic idea to buy rabbits though, i really do. It's amazing to have pets from a young age and you are right, it'll totally encourage responsibility - but i would take the lead, and never let them alone with them (i'm telling you what you already know, i don't mean to patronise!). I've just seen my LO stroke my Guinea's really roughly and it makes me cringe for them! It'd be awful if she really hurt them!
I know that Guineas should always have a cagemate, as they get so lonely, but i am not sure about Rabbits, i have never owned one, so i wouldn't like to say. I'd google it rather than asking at the petstore you buy them from as they'll just tell you whatever they want to get you to buy from them
!