The Latin Road is the program we used during the school year, but it's pretty intense. The first year takes about 1/2 hour a day (second year 45 minutes, third year a full 1 hour each day). It also requires some hands-on Mom help in the beginning. That alone would have made it a no-go for MY summer program. MY summer program was about time off for ME! (I am a certifiable wimp sometimes ... ) On the other hand, foreign languages really do benefit from 15 minutes a day practice, and a little bit every day is better than a ton of time once a week.
You can certainly start Latin Road during the summer, if you're really into that. When you are weighing the pros and cons, let me set out another option for you: Rummy Roots. You can play the Rummy Roots game and More Rummy Roots for just 15 minutes per day during the summer, and by the end of summer they will have a LARGE number of Greek and Latin roots memorized. We did this before starting The Latin Road and it was very successful. It seemed like the kids had something to build on.
So consider lightening up this summer by using Rummy Roots for a fun introduction to Latin, and then start The Latin Road in the fall, when you're back to school in earnest.