You can travel with your Adult Pass at any time sure, even if they're linked to children passes.

The only important thing is that children passes must be added in your basket when you order the pass. Not possible later.

The kids under 2 years have to pay reservations?

No but they will not get an assigned seat (when required). They'll sit on your lap.

Of course when seat reservations are optional/non-existant, feel free to use a seat for them.

If you want to have the possibility to make seat reservations for a child under 4 years of age, the child needs to have a free children's pass.