my girlfriend says i can only see my kids once a week ?

So, if your girlfriend said you couldn't see your children at all - you'd never see your kids again?  SHE doesn't have anything to do with your relationship with YOUR children. You should definitely go to court to get joint custody - your ex-wife has no right to limit your visits to inside her home. (Unless you have bad habits - drugs, drinking, anger issues, etc.)

There should be NOTHING more important than your children - don't allow anyone or anything to stop you from visiting them.  Having a father actively involved IN their life is important.  Don't screw this up.

Herein lies your problem "i dont want to get any type of court involved".
How much are you paying in child support?

Well, your gf is a selfish, immature idiot, but you're not much ahead of her.  How can you type out that your kids love seeing you, and ask a question like this?  Also, if your kids love seeing you, why are you not getting courts involved?  Do you love them at all?  If you do, be a father to them.  It's already traumatic for them that their parents aren't together, so this gives them a crap start to life.  Now you're letting your ex dictate when and how you see them?  I don't think you see how that comes across. 

If there are no courts involved, she could move cross country.  Guess what else?  Anytime she has those kids, she can refuse to hand them over -- ever again.  

I have no idea what's more important to you than your kids, but you have got to get legal protections built in so you don't rely on an ex for access to  your kids.  Dump this loser chick and start being a dad. 

I would say it depends who owns the house.
Either way, perhaps you need to re-evaluate your relationship. Your kids are a package deal and your girlfriend should have always understood that. If she can't accept them, why are you still sticking around?
Your kids come first. That shouldn't need to be said.

Tell her you can't accept that and try to work out a compromise like "I'm yours all weekend but Monday thru Thursday there's a good chance I'll be visiting my kids after work and not be home until 8:30."

A person who can't accept that ones young children will always come first is a person who isn't a good match for someone with young children.