I'm a single parent too, and dd and I have yet to take our first holiday together. When we do, I imagine we'll either do something with friends, or will go to a resort with a children's club. Mark Warner, Scott Dunn Holidays, Club Med and others have resorts/packages that include a wide variety of activities (suitable for all ages/skills), and also offer a childcare option.
Investigating and organising activities for kids (and you) when you're away from home can be tiring and stressful....and then there's the constant decisions about where to eat, how to get there, etc, when you're away from home. That's why a single destination with all activities and food laid on appeals to me. Also, when I did this sort of resort holiday (Club Med)as a single person pre dd, I found that people were quite friendly and so could easily socialise if I wanted, or be left alone if I preferred that.
Other single parent friends swear that cruises offer the perfect mix of activity, relaxation and good food - but I think your children are probably too young for that sort of holiday.
Get yourself down to a travel agent's office, and do some investigating. I bet you'll quickly find something that appeals. Good luck. Holidays can be hard, and I think you're wise to plan something as a special treat for yourself and the kids.