By "seat", do you mean the dinette seat? Do you have a couch opposite? I think usually the intent is to clamber up from the arm of the couch with that layout, which is rather sturdier than the dinette back often.
I just jump up from the floor myself. Depending on the ages and stature of your kids, I may be a good bit taller than them, which obviously helps with this—but for someone reasonably spry and not too short, it's more a challenge of nerves than of athleticism. I do also have a light little wooden ladder that can be leaned up and climbed, and can get tossed in the cabover when traveling, as supplied by the previous owner (who I think cut down a ladder from a household bunk bed or something similar to make it).
Ikea has a couple of decent looking little stepladder/step stool units that may fit the bill as well. Lots of options, as BB_TX astutely observes.