It sounds like she didn't get a lot of attention in her former life (even possibly a mill dog.)

She may or may not ever come out of her shell. Give her a few months to get used to things. You're doing the right thing by having your kids take care of her. Can you have them feed her too? (Make sure that you are not free feeding. A dog that age should do well with just two feedings a day, but for her size you may want to make it three times.)

I rescued my German Shepherd when he was 10 and it took him a long time to learn to play with anything than the toy he had his whole life. It took him even longer to accept that i wasn't trying to take his spot when i sat down with in a few feet of him (it took at least two years to be able to cuddle him if he wasn't standing up.)

Maybe try taking her to an off leash dog park so she can watch the other dogs play and interact?

Wish you luck!