Hi - we've been taking our DTs abroad since they were 8 months. As redblanket says, there is deffo an element of 'change of scene rather than a holiday' - that has changed now they are older and we go to places with kids clubs so we can get some down time and they can do stuff with other kids.
When ours were babies we did Eurocamp to Brittany as we figured it would be easier to be self catering - we stayed in mobile home not tent (largest available). One of the places we went there were 7 other sets of twins there! They supplied travel cots, high chairs, baby baths (check for their special toddler friendly sites). We took our bouncy chairs with us. Ferry was good - could wheel them round in buggy, we took play mat for them to play on floor and we had a cabin as well with travel cots in so we could all have a nap.
Pros and cons
Pros
easy travel on ferry
not too expensive
not too long a journey in UK or France (2 hours to site)
great swimming pool (go to one iwth indoor pool as well in case of bad weather)
usually have kids play tent with toys etc for change of scene/amusement
near beach
very family friendly
childrens couriers will babysit for you in the evening so you can go out child free
cons
self catering - we made it easy by french bread cheese/ham for lunch, bbq in evening
no child free time (except evening babysitting)
in bad weather can be very limited with things to do - weather unpredictable in brittany
we also stayed at an apartment in spain for a week which was owned by people who had twin grandchildren. large apartment, balcony, pool, fully equipped for twins it had everything, near beaches. It was not picturesque spain it was more golfing spain but it had everything there and was great for a week by the sea/pool. booked it through baby boltholes it was called Roda Golf Apartments near Murcia.
Since DTS are older we've done neilsons which has been brilliant for us all (Greece, Turkey) and we've had plenty of child free time whilst they go out on boats etc
Have a great time wherever you go!