In April, the Canaries with a heated pool in the hotel pool is probably the way to go if the pool is important. The sea in both the Med and Canaries will be too cold to be comfortable for most people.
I never do all inclusive because I don't like buffets for lunch and dinner and wouldn't want to be tied to food if it's not to my taste - I know you can eat out, but it makes it expensive compared with food you've already paid for as part of the package. However, I don't know what covid has done to the AI hotel buffet.
Whether AI or eating out works out best value depends on what your expectations are food wise. If you're big eaters or drinkers obviously AI can work out cheaper, but we usually compromise in that we'll eat out for a proper meal once a day, and for the other meal have a snack, ice cream, or easy supermarket food (cheese, ham, bread, crisps, olives etc). I couldn't eat two main restaurant meals every day anyway.
Or sometimes we go B&B and have a big late hotel buffet breakfast and then either a late restaurant lunch - works well in Spain where it's perfectly normal to have lunch at 3/4 pm and tapas or snacks in the evening, or have a snack lunch or just an ice cream and then a proper evening meal out.
What is it about Greece that is more attractive to your DC than a hotel in the Canaries with onsite heated pool and likely better weather? Is there a difference in what you can get for your money at either location?
We've always found eating out in the Canaries to be very reasonable, drinks quite cheap. Don't know about Greece, but I think you can get local food quite cheaply in many places, but then some islands, eg Santorini, have a reputation of being expensive.