The most important thing is to try not to make reading a chore. Incorporate it into part of the day, and make it a regular activity. It's best if you can do so in the mornings, so your children's afternoons are free for summer fun. The same goes for math, spelling, etc. If they feel like something is a structured chore — like it was during the school year — they'll be resistant. But if they're rewarded with summertime freedom for the bulk of the late morning and afternoon, then they'll be more likely to stay engaged with their summer learning.