honestly not far off i imagine he's learned to wear cheaper underwear during zhongli's rut bc zhongli has a history of just tugging childe's pants down, ripping through his underwear, and going to town (childe has 0 complaints)
i don't doubt he does have a specific collection just for that purpose (and he totally buys panties for it just because they're easier to buy in bulk/cheaply totally not because he likes that zhongli gets more excited when he sees that that's what childe is wearing)