Dog parks can be out of control, that is where I would NOT go to find a dog friend. I'm not really keen on the idea of dog parks although I go to them (I know, hypocrite) because of the lack of control everyone has over their dogs. I always have my exit strategy ready.
I would let friends know you are looking for this instead. I wish you were closer because my golden retriever would be perfect. In fact, most golden retrievers would be perfect. Big, hairy, overly friendly and kid friendly is kind of the hallmark of the breed, if you can find a typical one, that is the kind of dog it would be nice for her to get to know. Mine just expects kids to love him and pretty much wins them all over. Sometimes labs would be perfect especially older ones. And those two breeds are common. But a nice old mutt would be good too, just anything big.
I teach kids to stand with their arms crossed and look up and put their nose in the air like they are "stuck up" I tell them, if they want the dog to back off and it is invading her space. Dogs understand that concept very well because they use that with one another. They consider the space between their front paws "theirs" so teach her to be careful when dogs have toys and such in that spot, and they do not allow "lesser beings" to invade that space. They will not necessarily think she is a "higher" being than themselves unless she shows them this. I tell kids to "make themselves big" and remark when dogs put their hair up that they are "making themselves big" and tell them that to dogs, big means tough. But mention not to take things away from dogs unless they can give them something better, at least for now.
Another point for kids to master is how dogs learn. They learn by being told something over and over and over and having something good happen at the same time. Or being told (or shown) something over and over and have something bad happen, if it is a behavior you don't want. This makes it easy for kids to understand, very simple. But it gives you the chance to bring up what to when the "something bad" needs to happen. Children should always be taught not to hit dogs, as that is a sure way to have the dog bite them. They are better off using a scolding tone or grabbing the collar (if they are tall enough) and not swinging arms around if there is something really "bad" going on. I taught my kids that if they thought the dog needed to be punished, it was time to call ME. But kids can do the positive reinforcement part all the time. And a "punishment" to a dog might be just you withholding your attention. I tell kids...what they hate is to be ignored. If dogs are mouthy, for instance, I tell them to stop petting them or looking at them until they stop. That is a "punishment" to a puppy. In reality I do punish dogs, but I give them a shake on the scruff and would rather that be left to an adult.
Once she is doing better with some really nice dogs you could move towards younger bouncy bigger dogs. As far as where to find them, you might see if there is a local 4H club with some dog people who might help you out. Most people LOVE to share their dog with people and have others enjoy their dog so don't be afraid to ask. Or your vet might know someone with a trained therapy dog or a Canine Good Citizen. People speically train their dogs to be good with people who need company, they might find this to be something worthwhile to have their dog do. I would.
A real easy trick for her to teach your sheltie would be speak. Tell her to say the word "speak" every time the dog barks. After she has done that for awhile, have her hold up a small treat and say speak and if the dog does, get SUPER excited and give the treat immediately. Usually once that happens once they get it. If they are around another speaking dog they get it too. Shake is also really easy for them to learn especially if they tend to paw for attention. Kids really love to show off dogs doing tricks. Even playing dead is actually really easy to teach dogs, I could describe that one, but speak and sit and shake are better starter ones. Shelties are often the kind of dog that likes to do fun little tricks with kids. I have a collie that does tricks very well, my sons have really enjoyed that.
Then you can tell her she is "good" with dogs and a natural and all that mommy stuff we say to boost them up. Kids and dogs learn the same way so they work well together, small steps, short sessions, and when she gets a successful moment, tell her to "stop on a good note."
Hope that helps! That is what I tend to say to kids when they are adopting a young dog from us or I'm worried they may be a little inexperienced. And if the dog has a particular issue, I tell that child how to deal with that issue, not learning tricks. But it sounds like you have a nice little dog Dace for her to get comfortable around, you might just let that dog do the convincing for you. Not everyone is a big dog person. Having a great relationship with one great dog is better than many with various results.