Ok, so she doesn't want to be with you, doesn't want to give you access to your child, doesn't need your help, yet you aren't allowed to go one a ONE day festival?
Sod that! She is dictating to you & trying to run your life. Yes, her life has changed, but she cannot dangle the chance of a relationship in front of your face, like a carrot. Either she wants to try, or she doesn't.
It's time she accepts you aren't together anymore & lets you live your life, allowing you access when it is mutually agreeable. Or she tells you she definitely wants to try again. At which point, it all becomes about the three of you.
If you are free the day of the festival, go. If she agrees to access, do that instead, or if she needs help, help her.
But seriously, don't live your life waiting for her to tell you to jump! Because it will get higher & higher & you will fail each time.
Being in a relationship is about give & take, my dh spent several days abroad (long haul flight) recently, by himself, doing his own thing. He planned a trip to do this specific thing & he did it. It wasn't my idea of fun & I didn't want to be away from the dc that long, so I stayed at home with the kids, missing him terribly. I know that when something comes up that I want to do, I will do it. He will stay with the kids.
As an aside, we spent three days together before this, sans kids for our anniversary, so it isn't like I never leave the kids.
This is how we have been together for 25yrs, give & take. He has his stuff, I have mine, we have ours. Neither of us dictate what the other can do, neither of us expects the other to jump to our tunes.
Wanting to go a music festival doesn't make you immature either, who ever it was that told the op to grow up!
I've told her I'll not go and it wouldn't faze me If I don't go but she's telling me to go and probably meaning the opposite.
Sorry, but I cannot stand people who say one thing & mean another, then moan when their partner/whoever gets it wrong. She is a mother now, yes, her priorities changed, so did yours. If she wants you around to do x,y,z, do it. If she says that she doesn't want you around & there is nothing going on in your life, that concerns her or your dc, then go to your festival.
I cannot understand people who play games. I certainly didn't get through 25yrs with the same guy by playing games.
She also has trust issues (I've never cheated but she cheated on me early on in the relationship kissing someone else) so I don't think me going to a festival would help that. Look, I am sorry to say this op, but NOT going to the festival isn't going to help either! If you continue along this vein, every time you want to do something, she is going to give you a reason (valid sometimes, sometimes not) that you shouldn't do it.
If you should get back together, you will resent her constantly stopping you doing what you want. I am not talking lads weeks away, spending $$ on stuff, putting her & dc last, I am talking small things, occasional things we all need to keep us sane! 
I am not saying for one minute that you should act like you are carefree & single, if & when you become a family unit again. At that stage you base all of your decisions on what is best for your family as a whole.
BUT
If you remain single & things don't work out, at times when you are not with your dc she can't expect YOUR life to stop. YOU planned this festival at a time you weren't together. It isn't as if you have planned something for EVERY day, of EVERY week in the next few months, so even of she did need your help, you both could have worked out a way to make it work, around your commitments.
After all, what would happen if it were a work trip? Would you cancel that if she wanted to try again?
It stinks of her not wanting you, but not wanting anyone else to have you! She wants you to be tied to her, when if you aren't together, it is going to be virtually impossible.
If you were a woman posting this, you would be getting all out support for 'not waiting in for him to make up his mind' as to whether he wanted a relationship or not & being thoroughly encouraged to do exactly what you wanted to do.