Climbing into bed

I know a lot of parents decide to transfer their toddlers out of the crib into a big kid bed when they start to climb out of the crib on their own. I have been lucky enough to not have a child (yet) that climbs out of their crib.

I DO, however, have a child that likes to climb into the crib. Eli used to climb into the crib all the time. If he got tired enough while playing he used to get up and climb into his crib and take a nap. He no longer sleeps in a crib, so now he climbs onto the bottom bunk and naps. But, now Anna sleeps in the crib (at least for the last 2 days.) The crib was never taken down. it has always been in the room, but once Eli got his new bed he showed no interest in the crib... until now.

This afternoon I put both Eli and Anna down for a nap. Awhile later I heard the telltale noise of Anna being awake, a gentle thumping on the wall (I presume she is kicking it.) So I told Kiah I thought Anna was awake and Eli probably was too. So she went to check.

Kiah: Mommy, Mommy, Eli's in trouble.
Luna: what's he doing?
Kiah: He's in the crib.
Luna: Oh man!

So the big question is now.... How do I stop the 2 year old from climbing into the crib with the 9 month old? They were both as happy as could be int he crib together. My concern is Eli isn't really aware of his size. He is a big boy; reminds me a bit of a Great Dane puppy. Anna isn't crawling, nor is she able to get into a sitting position by herself. So my concern is for Anna's safety. Eli is VERY protective of her, but he is only 2, so his understanding of safety is limited.

Goofy Kid!