yes do it , my daughter took her shetland for 3 hours hunting when she was 2 , you arent going to risk your childs safety so dont worry one bit about anyone else and just do what you feel happy with, i regularly exercized my arab stallion in hand with my daughter 'riding', unless your horse is enormous so you cant grab her ankle i would not worry one bit!
My kids have been sat on horses since they were tiny but never independently. Ryan's 3 now and I still don't think he would have enough balance to ride on his own although he does ride granny's horse while sitting infront of her.
It's totally up to you as you know your horse and child best but I wouldn't think it's very safe.
I would think the biggest problem you would have with your daughter riding your cob is that as a heavyweight he is presumably quite wide so would be uncomfortable for her little legs and hips. (No disrespect meant to your cob btw!)
Is there another Mum at the yard you could swap babysitting duties with so you could both ride?
I am the unfit Mother who used to lead her pre-school son on his Sec A off her Highland and used to have to hold onto his top as poor little tot used to fall asleep!!!!
I have left them in the car with the iPad while hubbie has shod the horses before and we ust to lock Ryan in a stable with his toys when I was grooming the youngster on the yard lol
I was also going to recommend the DVD Players in the back of the car however if you are going to ride I would only do so if there are other people around on the yard who don't mind keeping an eye just in case you had a fall or something. Personally I only managed to do this once as my son would inevitably need the loo as soon as I'd got on!