mama's can raise their babies, but you'll have a better rate if you brood them  yourself.  this time of year, when it is this warm, simply a 100w bulb in a heat lamp would suffice, and they probably won't even use that.  i've got mine on, but they're never under it.
i've got a banty/lf cross chick that was getting trampled on in the middle of the coop floor by the hens and rooster (all the hens, even the broody), was just laying there, mom was making no attempt to brood it.  it was barely hanging on, and as soon as i saw him i brought him up to the brooder.  he's doing fine and is the quickest and jumpiest of all the chicks.
i was letting my hens raise 3 chicks they hatched, but one (for no reason that i can see) just wound up dead and half eaten by the hens.  brought the other 2 up to the brooder.
hens can raise them and do a wonderful job, but if you want to keep all your chicks, brood them yourself.  you'll be sure they are eating and drinking fine and well protected.  sometimes the hens can be a bit too discriminatory with their chicks