I'm so glad that you took him in and he has a loving home now. It sounds like he picked you to be his new family, smart pup!
We've raised all three of our kids around Pyrs and it's been wonderful. Pyrs by nature are very protective and generally very gentle around sheep, goats, etc. so in a home situation they take on the children as their charges to love & protect. As everyone's said, it's always important though to be watchful when kids are around any dog, even our own. That being said though, our pups have been wonderful with our kids (and visa versa).
In answer to the question about teaching him to trust & walk on leash with you. I'd give him a little time to settle in (few days or a week or so) and then put the leash on him and let him just wear it around the house for a little bit at a time, since he may never have been walked on leash before, who knows? Gradually, I'd offer him treats to walk toward you on leash and then slowly get him to walk on leash in your yard and then outside your fence, eventually down the street with you. I'm sure that others here will have more suggestions for you too, but that's what came to mind for me.
Oh & welcome by the way!