I've check both L and W's profiles. Both have Snapchat allowed in Playtime and both have a significant amount of Playtime set during their rest days which would allow them to Snapchat during this time. Can I confirm whether you mean that you set their profile to Sleep or Rest?
Rest Day (Day type): Weekend day with a more relaxed schedule made of usually Sleep and Play times only.
Sleep (Routine type): A routine in which by default, no internet content is allowed