First rule in a relationship if you love someone, don't cheat.
If she tells you it's ok, don't cheat.
If she has a hot naked chick waiting for you, don't cheat.
Like others have said, there are some big issues going on. You have a 4yr old and with that reponsibility, you are required to do the right thing. Frusterating, but it's just the reality of it. Work with her on fixing the issue. Have a date night and rekindle things gently. A visit to the doc may be in order.
I have 4 kids (3 - 13) and maybe, MAYBE get some once a week. Our youngest girl is 3 and sleeps with us and we know that hampers things. Thats ok with both of us and mutually agree to the decisions we made. We rarely have a date night and have never been without our youngest overnight for any reason. Yes, the love life is diminished and sometimes frusterating, but we both know what we were getting into when we signed up. I'll make up for lost time soon enough.
Just have patience with the situation and work with her on resolving the issues. Meanwhile, there's a fleshlight with your initials engraved on it.