My son was always and early riser and once he was in his toddler bed (@ 20 months because he was climbing out of his crib) he would get up and come in and demand breakfast at @ 4:30. It was terrible because we had a baby as well and between the two, I was up every couple of hours.
The only thing that helped was shutting his door and keeping his room pitch black. I think if he woke up and saw light, he thought, I'm going to get out and see what is going on! With the door shut, he maybe rolled over and went back to sleep? until @ 5:15...
I would childproof everything to the best of your ability,(including putting a gate on the stairs) and maybe put a baby monitor on in her room with the hope of hearing her wake up.
I wish I had a solution, but we find my two and a half year old doing all kinds of crazy things at 5 am - eating a loaf of bread! Calling relatives on our cell phones! eating corn bread! making himself a glass of water! putting in a dvd!
Good luck! If you find a solution, let me know. :)