Ok everyone, I've read your opinions and really appreciate them. I agree now that I'm not ready for this.
I thought I would just do a friend a favour and ride the ponies every Thursday for her, as her kids ride them at the weekends. Yes, she puts her kids on them D: I was quite shocked when they both felt so green and the kids ride them. I think she wants me to basically wear them out so they're (slightly) safer for the kids. She also told me yesterday eve that they take the 3 year old to lead rein shows and pretend that she's 5 so she can enter -_-;
Yesterday I thought it would all be quite light hearted but looking back on it today I know I'm not the right person for this. I don't have any experience with green horses (except one 4 year old that I planned to part loan that tanked off in a field with me, and the next day I said no thank you!)
Thank you everyone for letting me see sense.
I guess yesterday I thought it was sort of funny because they're just little ponies. It's like the difference between an excited Chihuahua and an excited Bull Mastiff.
I'll let the owner know today
Thanks again people