Tenting?

Today I set Isaac in his crib for a few moments to keep him safely out of the way while I had to do something complicated (I was trying to fix the DSL). When I was done (or, more accurately, when I had given up) I returned to his room. He was still sitting peacefully in his crib, looking at a book. When he saw me, however, he stood up, said "Down," and casually swung one leg over the top of the crib.

What?

It wasn't like he had to jump up to hoist his hips over the edge. He barely had to stand on his toes to get enough height. It was nearly effortless. For a second or two I stared at him -- one leg hanging over the rail, the other off the mattress but still dangling inside -- then I immediately overreacted, grabbing him with a shocked exclamation. Of course this got his attention and ensured that he immediately repeated the maneuver.

Thoughts raced through my head. The crib mattress is already on the lowest setting. Thank goodness he didn't try this while I was out of the room. He just turned 18 months old on Tuesday. The crib converts to a toddler bed, but he's still too young for it. I don't want to make a sudden transition to a "big boy bed." He is always cautious about climbing down from things, so he might not purposefully jump down. But if he got himself in an awkwardly balanced position straddling the edge, he might not have a choice. It's already 2:00 p.m. and his nap is supposed to start now.

So for his nap today I removed the metal platform that supports the mattress, setting the mattress on the floor within the frame of the crib. Although that makes the mattress low enough that he can't climb out, it's not safe. The crib frame is light and the mattress can slip beneath the frame -- he could easily shift one or the other, thereby making a gap between the two. I don't want him to go over the top and dash his brains out on the concrete floor, but I also don't want him to go under the bottom and strangle himself between the crib frame and the mattress.

So on his way home from work tonight, Craig stopped to buy a crib tent. Do these things really work? Any other suggestions?