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It will probably be cheaper to go it alone, I very rarely use organised excursions.
Perhaps do a bit of both? It's nice to have things organised sometimes when you have children and I'm pretty sure OV do some good family friendly excursions. But plan ahead as they tend to book up quickly. You can actually pre-book them on line at oceanvillageholidays.co.uk - you'll just need your OV booking reference number to log in - I think this is a great idea as it takes the hassle out of trying to book them once you get there.