man with child dating, what to do?

Seeing as my last post got removed for whatever reason, I'm posting it again but I will put more detail.....
I met a girl through some friends, my son August is 3 years old. I found out she works at the child care place my son goes to so she she gets a long with my child really well.
I got to know her really well, we were dating for around 7 months (only seeing each other) but were not official, everyone knew we were pretty much together, so eventually I officially asked her to be my girlfriend, so 3 months of an official relationship, all together 10 months.
Everything was going well until friends posted photos from a camping trip we all went on. The photos were of two of our friends, my son and myself. My girlfriend commented on it saying "Look at my boys, look at little Auggie" When my girlfriend said this she was referring to everyone in the photo, not just my son and me. She's very close with the two friends we went with..
My girlfriend has only ever posted one video of my son on IG, which she asked if she could, I have no problem with that seeing as it's on private.
My ex has gone way over the top about how dare she call him hers or even call him Auggie, or how dare she post anything that mention Auggie. 
I tried to explain the situation but got no where with it, eventually I cracked it saying "we had discussed this that when we broke up we needed to stay out of the other future relationships, you have a new man and I have no problem with that" as I think he is a great guy and has kids of his own.
My girlfriend has been ignoring her but my ex keeps threatening to try take my son away from me until I leave this girl.
Shes constantly messaging my girlfriend through other people.
My girlfriend has blocked her, even I blocked her on all social media but I need to keep my phone.
I'm considering buying another phone to use to message friends and just keep the one I have for when it comes to stuff with my phone
I don't want my relationship ruined.
Any advice on how to deal with a situation like this? legal advice?
If we got a restraining order how would that effect my son? To drop him off to the other we usually meet at the park?