NICE play house, what lucky kids! I think that's an excellent place for Guineas, and a really good use of existing space... plus very cost effective for saving on materials to build an actual coop elsewhere. Not sure how many Guineas you plan to house there, but a flock of 8-10 should do fine.
The only possible health risk I'd be concerned with is if your kids might have allergies to the dust/dander the birds can make or the bedding materials you will be using. Developing a deep litter system could help with the dust issue, plus once you get it going it's pretty easy to maintain and creates excellent garden compost. You can research the deep litter method here on BYC, there's a ton of info on the correct ways to go about it and the best bedding and organic materials to use/add to keep it working like it should.
You'll need to keep the flock penned in their new coop for at least 6 weeks before you let them free range your property, if you want to condition them to return to the coop each night to roost. You want them to imprint on the coop as home and a safe place to roost, and that takes some time with Guineas. They are not as easy to coop trains as chickens are. And after the initial containment period you will need to be consistent about herding them to go back in and making that the normal nightly routine for them. You may get away with less time needed for imprinting them on their coop as home and where to roost, but you will still need to be really consistent about herding them back to the coop each evening.
If you use treats for a reward and the same call repeated over and over every time you feed them/give them treats (from day 1) they will associate that call with food/treats and learn to come when you call them. Once they have a routine established they tend to stick with it... you just have to make sure you establish the routine for them, or they will establish their own (and may start roosting in the trees or on top of the play house, instead of under it). It also helps if you ONLY feed them in the coop... the more they associate the coop with food/comfort, the faster it helps them develop the routine. I've used this method for years, and at feeding time (for all my animals) I have Guineas waiting in or at the coops wanting their nightly treat. (Guineas are very food oriented, and like routine/consistency, and I always recommend using all of that to one's full advantage in hopes of having a more easily manageable flock. Consistency is key tho).
