Have you checked what size paintbrush or roller she can hold? Maybe even the smallest is too big for her.
If you find one- no point in doing part of a wall so maybe stick to skirting boards... or light switches if you have a ladder and safety harness or can hold her up.
Maybe just let her stir the paint or sandpaper the old skirting board or wash the brushes.
She's lucky to have someone trying to give her all these experiences at 6- at that age I done no decorating, horse riding, had no dog to walk...
She would be brilliant at doing the skirting boards, and places like under windows where adults get back ache from the bending.
She could Use a small gloss roller as it would probably be Ok in small hands