FIRST THINGS FIRST - introduce them to each other on equal neutral ground. In a place that NEITHER have been before. Even before getting to your house.
Second - Keep them separated by a door in the house for at least for a week, usually 2 weeks. Let them get used to each others scent under the door and on your clothes. Make sure that your daughter and you visit each dog as often as possible and switch to the other dog right after to have the scent fresh on you.
Third - Introduce them slowly in the house, keep to separate rooms when no one is home and at night.
Fourth - This is the point where they will figure out who of the two is the alpha/top dog. This can include everything from growling, nipping, stealing toys/food. If it goes beyond that go back to step two and start again.
Feed your dog first before you feed the new dog in your house. Everything in your house pet related (food, treats, attention) should be to your dog first.
Feed them in separate areas till they settle who the Alpha is. Bedding should also be in separate areas. If they are both into toys etc. in the house your dog gets priority to his toys etc...
You can get help from trainers, breeders and other pet related folk in your area BUT this is the way to start.
Make sure that you, yourself are NOT stressed about the new dog moving in, your stress will make things worse for your dog. What and how you feel you dog knows but not the whys. You're dog is your champion and whatever stress regardless of the reason your dog will want to protect you from it.
When the dogs meet for the first time let the non-aggressive dog sniff the other dogs butt first. I know weird, but in dog psychology it works this way 'The other dog is too busy sniffing my butt to attack - so ok.'
Also when you are saying "It's ok" to you're dog when he is growling that is feeding into his fear/aggression. Don't say anything, ignore him if possible or make a tsking noise, snap your fingers or something along those lines and ONLY for his bad behaviour. Talking, saying it's ok just re-enforces the behaviour.
AND if you're stressed about this meeting you're dog will sense it and behave accordingly.