Let me begin by saying that all relatioships have their problems and fights, especially poly ones because you are trying to balance three different points of views and lives together. If you are not able to handle having fights and coping with the fall out, then you are not going to be able to have any form of relationship that will be successful. Poly relationships are all about balancing lives and compromising and making things work for all of those involved.
You have mentioned in your post that they "celebrated" their marriage, where they should. You make it sound like just because they brought you into their relationship that they should not have a past and that everything before you should be unimportant. They should be able to celebrate their past without you feeling bad as long as they still celebrate the milestones within the poly relationship as well. If they refuse to recognize your anniversaries or dates important to the poly relationship, then you may have a problem to argue about.
As far as these dates that they go on, what exactly do they do? Do they go out every night without you and just expect you to be okay with that? If so then there could be a problem. However, if you have agreed to give them time to work on their marriage, which definately sounds like it could use some TLC, then you cannot be too upset when they try to do that. You should not say things that you do not mean in relationships because it only leads to problems and your resentment for their spending time together. Do either of them make an effort to spend any time with you or are they always by themselves? It sounds like from this and some of your other posts that the three of you are all very busy people, and that could play into some of the bad scheduling. Sometimes that is all a relationship really needs, a set schedule of things to do and when to do them.
Now you have mentioned that you love this little girl, but you make it seem like it is such a burden to have to "babysit" her. Do you consider yourself to be a part of the relationship and the family? That little girl probably sees you as a parent, as many children of poly parents do because they do not know the difference. While she is not your biological child, you got into the relationship knowing that the child was a part of the package. When a child is under your care and you and that child love each other like a parental child relationship normally is, it is not called babysitting, it is called being a parent. Do your bf and gf ask you to watch their child, or simply expect it and was it something that you had said was ok when you got into the relationship? You have said that you do not work or contribute financially to the relationship and that they are your providers, so having the responsibility of taking care of the child shouldn't be too big of a burden as long as they aren't just going out and excluding you every night. You say how much you want things in this relationship to be equal by acting as if they shouldn't have a past, but then you turn around and say that you don't want to have it equal when it comes to being a parent to the child, which if you didn't want that then they would have to hold onto some of that past that it seems you are so desperate to try to erase.
You also mentioned in this post that on top of feeling like a live-in babysitter, that you also feel as if you are your bf's sex toy for the things that your gf cannot do. What exactly would you be able to do for him that she couldn't? What is your sexual relationship like with your gf, because you have only mentioned your relationship with your bf, which again doesn't sound like you want an equal relationship. You make it seem as if you are only appreciated for the things that you are able to do in the sack and nothing else with your bf. If that is the case, then make an effort to do other things besides have sex.
I am mixed about this because you seem to be complaining about them having a past and then not wanting to move forward as being a part of the family and being a parent to the child. They need time to work on their broken marriage, and you agreed to that, but they do need to make time for you as well. You do not contribute to the household and seem to complain about having to do something as simple as watching a little girl, when they could definately require you to help with the bills. You need to do some things to gain your independance from the relationship, but you also need to appreciate what you have in it as well. You need to figure out what is causing you to feel like a sex toy with your bf and why you don't mention much about the relationship with your gf. In the end, get some therapy, couples and individual.