Kevin McNulty, one of Grandpa's old friends (and of course, related to Timmy and the McNulty boys), goes through the secret initiation in order to get into the Loyal Order of Wombats. After seeing this, the Rugrats have Dil go through something similar, so he could become one of the "Big Babies". In turn, their fathers mimic the Rugrats, on a bet that they could do everything the Rugrats do. (thetvdb.com)