Bonjour
Emmmm.... 2 sentences could be right but have a different meaning ........
Ils s'endorment tard. (3e pers. pluriel) they fall asleep late
Ils (elles) s'endort ==== FALSE
ils les endorment tard ===> they put them to sleep late
anyway can't be "endort" because the subject is "ils" = 3e pers. plural