You should still be able to get insurance, in the EU at least your dh can have any necessary treatment in an EU (public) hospital, there are also a number of specialist suppliers - my mum has cancer at the moment for the third time and has managed to get insurance.
How old are the teens - a 13 year old and a 19 year old are not into the same things.
Some relatives of mine took teenage dd (16) and dd's friend to Spain. They booked two appartments next door to each other and only saw each other twice.
Have a look at the insurance, and health care in general (I know of people on dialisis going to Spain and having dialisis there, in fact some move because the chance of a transplant is higher) if you can get insurance what about Mallorca?
OK I've never been, but I keep watching episodes of 'sun, sea and A and E'.
I hope you find something and have a fantastic time.
Oh one other suggestion - we got the Sun vouchers last year, and went to Haven. There was loads to do for teenagers and I actually think when you added up all the things we paid for you would get the same booking directly.
In the day there was swimming, archery, climbing wall, fencing. In the evening there was entertainment and another quieter bar.