Newborn and toddler? Help

How do you make sure your newborn is getting adequate sleep with a toddler around? My 3yr old will be downstairs or in her room with my husband and occasionally buss in the room and wake my 2 week old newborn I know for a fact he can’t be getting enough sleep because he doesn’t sleep longer than an hour without being interrupted and he’s up at night crying for 4 plus hours. I try to leave our toddler with my husband while I stay in the room with our newborn so we both can nap because I’m exhausted but she literally will come in super loud and wake him I know she doesn’t mean to but Idk what to do… I feel like I’m failing both ways.