Good for you for having a trial component...I think ALL families as well as nannies should have this because it just doesn't make any practical sense not to.
Just let her know that her being late was an issue with you as well as her not cleaning up after the kids. Let it be a learning tool for her so in her next position, she will know what her "weaknesses" are.
If you really have a ton of guilt, you can always pay her a little severance pay upon letting her go. But considering that she didn't do her job well, it wouldn't be necessary.
Good luck and I hope your dream nanny is around the corner.
