When our first child climbed out of her crib (and we literally caught her in the act, just as she dove over the railings), we took the mattress out of the crib and put it on the floor. Since I was pregnant with #2, we also felt that it was time to encourage her to grow out of the crib, if only so she wouldn’t feel like the baby was stealing it from her. A couple of weeks later, we got a toddler bed and a guard rail, and moved the mattress to the toddler bed. She liked the freedom, and never missed the crib, but when she left her room in the middle of the night, she usually just came straight into our room, and spent the rest of the night there. We didn’t have room in the bed for her, but we did make a nest of blankets for her to sleep on. She pretty consistently woke up every night and came into her “nest” in our room sometime between 2 and 3 am until she was six or so, even through three moves. (That meant we had the bedroom to ourselves in the evening, so it was a compromise we could all live with.)
Our second child, though, was a scare. I think we were fortunate in some ways that he had physical problems that prevented him from walking until he was nearly two, so he was in the crib until relatively late (almost three, I think) before he was even capable of trying to climb out of it. When he did finally start moving on his own, he was obviously trying to make up for lost time. We put the mattress on his floor, so he couldn’t fall out of bed trying to climb out, and didn’t even try a toddler bed until much later. We also had to turn the doorknob backwards so that we could lock him in, and we took virtually every piece of climbable furniture OUT of the room. We left toys that he could play with safely without supervision, to keep him busy. As others have said, we felt guilty locking him in, but 1) he would not have been able to get himself out of the apartment on his own in an emergency anyway, and 2) he was MUCH safer locked up in a child-proof room than he could ever have been roaming around the apartment by himself when the rest of us were asleep. He also could not/would not sleep in the same room as someone else. Even as an infant, if he was aware of anyone else in the same room with him, he would stay awake until he was alone. The kids had to share a bedroom for a couple of months right after he was born, and we had to make sure that he was SOUND asleep before we could put his sister to bed. Her coming to our room to sleep in the middle of the night meant that he would actually sleep later the next morning, too.
The mattress-on-the-floor turned out to be a boon, too, when he broke his femur at the age of 3 1/2, and had to wear a body cast (chest to toes) for a month. Since his mattress was on the floor, we didn’t have to lift him very high to get him into bed, and we didn’t have to worry about him falling out in his sleep. This kid was/is so active though that by the time the cast came off, he had learned to use his elbows to push himself off the mattress onto the floor, then push himself into the living room (sliding on his back across the floor) to watch TV until someone else got up.