Hamsters are lovely (I'm a convert since getting my kids one). The first one we had was probably the best hamster ever. She was called Timmy (pet shop mix up with genders- we discovered what was missing later...) and she was awake from about two in the afternoon. She loved coming out and would look at you with these really pleading eyes and stand on her back legs, looking like a begging dog. She chased people in the ball. She loved snuggling. She learnt obstacle courses and played on the bed. All our other hamsters have been perfect, none have EVER bitten us.
Hamsters, once bought, need to be left alone for 24hrs. No peeking from your 5yr old (or you 
). No coming into the room. No holding. Put the carry-thing in the cage, open, or gently tip it out, and then exit. Show your 5yr old the hamster, then tell them that they have to be really, really, really quiet for a day. When you start handling, I'd say you and DH should start handling first- a gentler person and also a 5yr old is generally more excitable about it, so the hamster might get scared. When you are taming it, let it sniff your hand, and, if after about a day of you having treats on your hand but it not coming, you could try to gently scoop it up in your hands, and pet it while it is still in the cage. From there, everything moves quickly and hamsters are tamed in about 3-5dys. When it is comfortable outside the cage, but still not as tame for a 5yr old to handle, let him stroke it and feed it food so it gets used to him, until it's used to its home and he can learn to handle it.
I would set a timetable for cleaning out. Every Sunday for instance, at a certain time. Your 5yr old will never be able to do all, or even most, of it. Have alternating weeks, so one week your 5yr old helps DH (even if it's simply by chucking away the woodshavings or replacing the bedding or even just holding the hamster when you are vaccuuming the room for mess- hamsters should never be in the same room as someone vaccuuming, they get scared) and the next week, he helps you. It really helps.
Petsathome are a good petshop if you are buying from a petshop, but if possible, get from a breeder or rescue. Never get a hamster which appears to be hyper, aggressive or dull. An inquisitive, energetic but not hyper hamster with bright eyes and a sleek coat is best. About 8-12wks is generally good, and be prepared for the hamster to die within 5mnths. For the first 2wks, check for Wet Tail, a deadly disease in rodents- if it is pooing a lot, has a wet patch around his tail for about 3dys straight or seems listless or isn't eating, I'd take them to a small animal vet- it's not too expensive usually, and they can treat it with antibiotics.
Goodluck with the hamster!