We took the Yanks Tour with our 9-year-old and 12-year-old. They are experienced bicyclists, but even they had a little trouble in the hot late-June weather. Fortunately, the weather had cooled a bit from the previous 95+ highs of the last few days; when we went, it was more like 85 degrees. Most of the rest stops are in the shade, but there is much bicycling in the sun.
Bruce was our guide, and he was knowledgeable and helpful. He stopped at a special place of interest for us, and took his time at the stops. Best of all, he had water and peanut bars available at the top of Little Round Top.
The adults in our party enjoyed the trip immensely, but the kids found it a little rough going. I wouldn't try it with kids who were not experienced on the bike, at least not in hot weather. Do bring more water and snacks than you think you'll need.