Link from another topic which might have some useful info -
Main thing is don’t get a plastic 70’s style board. Get something around 7.25" wide for your 6YO as they’ll probably end up sitting and buttboarding on it for the first while.
For yourself, look at something bigger, depending on your height, shoe size and weight, usually 8.25 - 8.5 will do the job just fine, with whatever wheels, trucks and bearings you can afford. Don’t go overboard on the cost at this stage as you won’t notice, or need, the difference between the entry level and pro level gear, which is priced accordingly.