I've remade for Sims 4 playpens I had previously done for Sims 3, and I've also included Sims 3 original playpen.
Of course, as we don't have real playpens in Sims 4, these ones aren't perfect but you can place it around children, and then, they won't get out from it until you remove it (a Sim can do in game that by putting the playpen into their inventory). Toddlers can still play or move into the playpen, but they won't go outside. They won't be able to make a mess neither, which is a good thing if you ask me! :D