Oo normal yan, lalo na pag baby pa lang kasi mas madaling respire through the mouth, especially kapag congested o tired pa lang sila! One of my mum friends actually says the same thing, that's her sign din na deep sleep na talaga ang baby niya, so don't worry. Though if it's paired with snoring or parang hirap siya huminga, better check with your pedia lang para sure, pero hanggang dito nakanganga lang mostly okay na yan.