Elf on shelf

My kids are almost 4 and almost 2 We have decided to just do 10 days of elf on shelf this year . I’m just wondering what is the best first explanation for the elf’s , like what is their story ? I’m just wondering if anyone who’s got older kids like how’s that story developed etc is there anything you wish you done different ? Any movies to watch etc New to this and want to get it right and cover all angles . I’m aware it can be what we want it to be …. My son already questioned how Father Christmas was going to fit down the chimney and I just had no explanation ready for him 🫣 I really don’t want to F it all up !! Ha I worry In all honesty bcz when I took my boy to pepper pig world he met the characters and looked disappointed or like atleast like his mind was questioning it for sure, I could see he wernt really buying it but I still told him they were real Thanks