Hey Liz I've just seen this. We needed to get S out of the cot as it wasn't coping as a trampoline! I was dreading it as she is really active in bed, takes ages to settle to sleep and generally spirited! So I posted here about it too, oh and as you know, asked on the birth club!
We got an inflatable ready bed that she LOVED, but the three nights we used it she rolled out a few times so I popped her back in her cot.
Anyhow last Sunday DH built her toddler bed and we went for it! She was unsettled initially but went to sleep quickly. The bed has built in guards at the head end but she flipped ends and rolled out the first night! I put her cot mattress on the floor to cushion the blow! Since then I've had a rolled up towel covering the gap. I tried her without it last night and she rolled out again
She's getting used to the pillow so hopefully she'll 'get it' soon!
I worried most about her escaping and running wild during the night, but no, she doesn't get out of bed unless we're in there! Maybe it's two years of conditioning making her wait for us or she just hasn't figured out she's free, but so far so good!
Most nights she has settled quicker than normal, tonight it took 90 minutes, but she never got up once, even to rescue Piglet who was just the other side of the bed!
I think she was suffering 'cage' issues and now enjoying the freedom!
I'd say give it a try, just stick his mattress on the floor, if it all goes wrong you can pop it back in the cot! At least while you're up half the night with M you can keep an eye on him!