Toddler Climbing in Newborn's Crib

My son isn't due for another five weeks, but I set up the crib months ago to try and help my two year old adjust to the change. She wasn't interested at all in the crib until yesterday. She figured out she can climb in/out and now it's a game to her. Does anyone have any tips on how to stop this? She is very stubborn and speech impaired, so it's difficult to see if she understands what I'm saying when I try to explain that it's little brothers bed and punishments do nothing. He will be in our room for the first few months, but I don't want this to become a habit, so trying to stop it before he gets here.