When I go out for a leisure bike ride with my DD (just turned 3) I put our small amount of bits and pieces either in my v small backpack (it's really my handbag) or in a small handlebar bag.
I ride to work once a week when I don't have to take her to nursery and I have a pannier that converts into a backpack, if I am cycling to the station to go somewhere for work I do the same.
I want to start cycling with her to work, but as she goes to the workplace nursery and usually needs various changes of clothes/food/raincoat and I also need my work stuff, notebook, papers I've been reading at home, bike lock (annoyingly won't attach to my bike without swinging around and was VERY expensive), I can't fit it all in a small enough backpack or the handlebar bag. And my pannier won't attach under the toddler seat.
I have a hybrid Trek - what could I use to get my and her stuff to work? I can actually fit her changes of clothes etc with my stuff in my pannier, just can't fit it on the rack when her seat is on there!
Also prefer a cheap option as my pannier rucksack is lovely but was not cheap!