Two of my kids are sickle cell carriers, but how is it possible that they are not affected by those traits?

Nothing unusual: Most children with sickle trait are asymptomatic other than their hemoglobin level are slightly lower.

Trait is protective: Sickle cell trait has evolutionary value in that it protects against malaria infection. Trait people are well, and don't succumb as easily to malaria.

Metabolic work: In many conditions there are silent (unaffected) carriers. Each cell should have 2 copies of gene information that works like an instruction sheet for cell function. In sickle cell, the presence of one correct gene has enough information to tell the cell how to work normally. The bad gene is in silent.If you get 2 bad genes, you have sickle cell because the cell does not know how to function.