Well, maybe for the 18 year old, but that probably wont be suitable for the younger lads.
They sound good ages for the lads to appreciate the trip on the boat, and have a good time messing about in kayaks and dinghys, while you sit back and keep an eye on them from afar, while sipping a glass of wine (or gin), the eldest sounds old enough to take care of his younger siblings, just remind them if they drop their phone the chances of finding it in the mud are slim. Just ask London Rascal whose partner dropped her waterproof iphone off the boat, and it kept pinging her watch with messages for 3 days.