On websites, the problems are presented randomly, according to their difficulty levels, but not grouped by topics. So it's not easy for beginners to figure out which tactic to use. Sometimes the problems may require several tactics.
Therefore, I prefer books to websites, for beginners to study.
Another reason I like books is more psychological. A book has finite number of problems, let's say 1000, your kids can finish several pages a day, and finish the whole book in 2 or 3 months. It's a big achievement, and positive feedback, which will give your kids great motivation to study more. Websites don't have this benefit at all, in fact, the never-ending problems will make anyone despair.