I got my 3yr old son the most perfect horse. Except for one thing, he can't do anything with her on the ground(learning to brush, saddle, all that type stuff). I knew when I bought her she had an issue pulling back. I've done everything I know to do to try to fix it or at least make it better. Even if she isn't tied she'll still do it. It's not just a step or two. She runs backwards. Previous owners told us she was abused so I'm sure this is a result of that. She's so perfect riding I hate to get rid of her because of this issue but I can't trust her around him on the ground. Any suggestions??? 

Get rid of her. You won't fix the issue and you don't need to mess around and let you son get run over. There are better ones out there for a 3 year old.

Edited: At 3, he doesn't even need a full charge backwards to be severely injured. All she needs to do is a couple of quick steps to check the tie ring everyone will suggest and your son is on his way to the hospital. It's not worth it. 

Buy her a companion pony that he can work with and you take care of the ground stuff for him.  He can just ride her. 

One thing to try, get her a pen at a trailer.  Sometimes being boxed in with protection from behind can help a horse feel more secure.  It worked for my old horse. He had several trailer accidents and would pull back until he was free, whether it meant breaking something or coming untied.   I didn't have a 3yo kid around him, but I wouldn't have want to.  He squashed me a couple of times even though he wouldn't hurt a flea the rest of the time.  

Otherwise, you should probably send her down the road. Trust issues can be exceptionally hard to break and it's not worth getting a kid hurt over. 

I have a horse that does that and I would not want a small child around him while being tacked up. I knew he sat back when we got him and as an adult just starting out in my horse journey with a trainer; and I was able to manage it as he really only did it a few times a year. But it has got much worse as he got older (now 22). He will sit back/run backwards until he breaks something..halter, trailer, tie ring, etc with no regards to what is behind him. He will do it UNtiled when he thinks he is going to be boxed in i.e. too close to another trailer, etc. I got a high tie, modified it, and that has definitely helped as he freaks out if he can't put his head down but occasionally he still does it on that if something freaks him out that he didn't see coming. I have had this horse 8 or so years so I can read his behavior and know what type of mood he is in and adjust accordingly. Every other thing about him is perfect but I don’t have kids and most certainly wouldn’t want a small child messing with him while I am tacking up.

Not a fix for everyone but I have had some luck with trying to an intertube or an easy now by chute help. 

I'm sorry but cutting your losses and getting rid of the horse is way cheaper than a hospital bill. If it were an adult I would have other suggestions but a very small child doesn't know how to get out of the way. 

This is NOT the perfect horse for a 3-year-old child.

One that panics to any kind of pressure on the lead rope, whether it's tied up or not?

How will you lead your child around on said horse? A 3-year-old child is NOT going to ride themselves.

No. Just absolutely no. A reactive horse like this is not suitable for a child. If there is this large of a hole in her training, there are bound to be others. The safe "steady eddy" horses for kids are just that: safe steady eddy's. Not ones that freak out.

Sell this horse and get one appropriate for a child. You'll never see your life flash before your eyes like you will if you see your child headed for the dirt.

What happens when this horse hangs its bridle on something or reaches down for a bite of grass while your son is riding and steps on the reins?  Things like this happen all the time and if the horse has a freak out, your son could be badly injured.  

Setting back is a hard issue to fix. Expecially if it is fear/anxiety that is causing it and if she does it not even tied up. I don't think this is fixable. But at 3, finding a nice pony or donkey closer to his size might be a better option if you are wanting them to learn to groom, etc. But I'd think at 3, no matter the horse, I'd be keeping them back because anything can happen to quick for you to react. Even the safest bomb proof horse can half an off day. 

Sell the horse and buy a good pony one that has been there and done it all with small kids....I know a lot of people allow their children around horses....I don't too much can go wrong to quickly and have deadly results.  

Ponies were made for kids...they can lead, brush, and even fall off....safer from a pony...

I would send her to a problem horse trainer for a spell and complete evaluation. If they are unable to fix, have them market the horse for you.

The horse in my avatar pic was the most phenominal horse under saddle in every way. Probably one of the most broke and finished horses I've ever owned. However he was extremly watchy on the ground and would set back. I sold him when I was 8mo pregnant. I was not about to have anything on my property that I couldnt trust around my child 100%. There are so many good ones out there to risk a child's saftey for a **** one.

We decided to get rid of her. Old owners are going to take her back. I honestly have the perfect horse for him already. She has zero holes anywhere. He leads her all around the fields(I'm with him of course). She's just so light mouthed until he learns to be gentle with his reins(he's still being led anyway) he just won't be able to ride her with a bit.