I'd keep them in for a bit - they are very vulnerable when small - cats, birds of prey, crows and magpies.
Absolutely also weasels, rats, dogs, foxes, anything passing loves a chick. The mother will do her best but they will be picked off one by one. We usually wait until they have at least got their full feathers, but even then I prefer to keep them under cover of some kind - usually in my polytunnel destroying the veggies

until they are about half adult size We have even had rats digging under the coop to grab ducklings from beneath - hence the terriers we now have so no rats come near, but we also make sure the floor of any coop has fine mesh chicken wire covering it.