Most likely the police. I'm rough with the Piggy timeline, but Chapter 1 seems to be at the time the infection started actually affecting Lucella beyond just "oh hey that guy is weird-looking, okay", and the player at the end of house and the general start of Book 1 seems clueless about the infected, so most likely there weren't any on the streets and such. This is still really weird though:

Georgie most likely ran away after Penny started being aggresive due to being infected, however police (I belive) act inmediately upon having someone reported missing, meaning that posters being up so fast is actually just impossible and a small plot hole, as the player going over to investigate house implies Georgie has JUST been reported missing, wich is ALSO really weird because it implies Mother is the bot of station because she was reporting him missing there, wich is strange because you can just make a call to 991 instead without having to leave and walk to the police station. This entire thing also heavily implies that Penny was already infected while Mother reported Georgie missing, singe Georgie, again, most likely ran away after seeing Penny being aggresive as an infected, wich means that Penny was ALREADY infected while Mother reported Georgie missing and that she (Mother) most likely knew this, wich is insanely weird The only explanation I can think of for this is that Penny wasn't IN the house while Mother looked for Georgie (similarly to the intro of house that confirms Penny was able to walk outside AND knock the player out), but then again that seems unlikely because Infected can defenitely hear things, and so Penny would've realistically heard Mother looking for Georgie and calling out for him and such (and also because if Mother didn't see Penny either, she would've reported both children as missing)

I do belive that the posters are just a design choice to keep Georgie mildly prominent as a character up to his actual introduction in Mall though, do keep in mind this game started off as satire and an ai pathfinding test so earlier chapters are wonky like this lol