Headscratchers / Sandcastle

• There is a thing about the book’s ending that is confusing me a bit: at the end everyone except Zoe and Louis’s daughter, who is 50 in the end, had died; while this is perfectly justified for the former adults it’s a bit confusing for the former kids. In the book they arrived at the beach at 8.01 am and they were respectively 3 years old (Felix), 5 years old (Zoe and presumably Louis) and 14 years old (Sophie) and (doing a quick calculation based on the fact that in the beach you age one year every half hour, which mean in one hour you age 2 years, then it would imply that they would age by 48 years by the next day) it means by the next morning at 8.00 they should be 51 years old (Felix), 53 years old (Zoey and Louis) and 62 years old (Sophie). Taking in account that Louis and Zoe had their baby presumably when they have reached 20 years Old (8 hours After their arrival, which Is 16 years) it means that she should be 50 the next day at 17.00, yet her parents and relatives (the former young ones) are already dead respectively at 69 (Louis and Zoe), 67 (Felix) and 78 (Sophie, who effectively is already over the death of old age range), which, while plausible in some circumstances, it’s barely minimum for dying of old age. Of course there could be other reasons for their death (some could fall on the fridge category) yet what do you think?

• Why did the Inspector's phone get disconnected? And why was someone both spectating the beach from afar, as well as shooting anyone who comes close?