We have 23 month old twin boys (I'll call them C&D) whom we have about 2 months ago transitioned away from a crib by taking away one side and creating an enclosure of sort (about 12 x 12 feet) which they can busy themselves with so they're not climbing out the other side into the adult area. We have a pretty established bed time routine from since they were small. However what we have noticed happening recently is that C would walk over and start riding on D like a horse, possibly hurting D and causing him to cry. This would happen soon after we leave the area (we have curtains to separate their area and ours), several times per evening. I tell C to say sorry to D and tell him not to do it again etc. nicely and with a stern voice but obviously it's not working.
Can anyone recommend a good way of discouraging or dealing with this behavior?