I am a little confused when you ask about children's activites but then say you do not want to put them in any "programs." That is what children's activities are by definition.
Crystal has many "adult" activities onboard, but they will likely be considered a little to scholarly for the average 8 or 10-year-old. Unless they want to learn to play piano or speak French.
The children's program has Fantasia, the children's playroom, which offers tables and chairs for arts and crafts, Sony PlayStation game titles, a large screen television set and board games. Crystal Symphony and Crystal Serenity also feature a teenager's video arcade, Waves. The Library offers children's books and children's videos or DVDs for enjoyment in staterooms.
The dining is sublime on Crystal, and they do offer special dishes. The outdoor grill has hamburgers & hotdogs.
Crystal has programs for children onboard, I suggest you use them. here is a link to the information: