None of the children live on the beach as far as we know - the smaller children's father does but as far as we know his own children don't live with him, given he's known his girlfriend's children 3 months but his children first met them this week.
So there's no reason to assume that the children - any of them - are used to risk assessing safety near the sea.
Half the UK coastal drownings each year are people who are swept or fall into the sea rather than people swimming.
It doesn't say which of the teens took the smaller children to the beach but it does say it was just one of them.
5 and 7 year olds can be either very placid or utterly feral, or anything in between. My oldest boy is a pre teen but I've got a nearly 14 year old daughter who has an 8 year old brother and yes, like most mothers I think she's pretty excellent all round and mature and competent, certainly adored by small children as most friendly teens are. I'd be very unwilling indeed to let her babysit a 5 and 7 year old she'd only just met on a beach without adults on the beach too. Especially with cold UK late spring water which makes drowning if you find yourself in the water unexpectedly highly likely even for people who can swim.
Imagine living with having been in charge when a smaller child drowned. Imagine being the mother of the heroic young teen who drowned trying to rescue a 5 year old you'd just met.
It's pretty terrible parenting sending a young teen to supervise a 5 and a 7 year old they'd just met on a UK beach without adults near by. As others say play football in the garden or park depending on the local traffic and logistics because of supervising them crossing roads and how feral the smaller ones are, but not by the sea!
Supervising a sibling is a little different because the teen knows how reliable or otherwise the smaller ones are, but these children were meeting for the first time.
The AIBU was AIBU to tell exH I don't want DC to be left with his DP's teenage boys.
It's a shitty situation that where parents are divorced crowing over the father's sovereign right to do any irresponsible shit he likes as long as it's not illegal trumps safety concerns. Tell might be confrontational and therefore escalate the situation spitefully, but have a diplomatic conversation about not relying on a 14 year old to supervise the smaller children on the beach!
The concern might have focused on the wrong thing but it doesn't mean the children should be in the sole care of a 14 year old they've just met, by the sea.
As for abuse - that's probably more likely to happen inside the house than on the beach given the children are all under one roof.