Euh? Each venue will have its own pricing and rules - what is the issue? You dont like the price? Dont go? Of course you can try lye your way in and teach your kids its ok to lie to get your way
I don't recall our son's ID ever being checked (he is now 11), and we've gone to Europa Park almost every year (sometimes twice per year) for the past six years or so. I think the only time his ID was checked was when we went on his birthday. But I personally wouldn't risk it and would just pay the correct price for your child's age. Besides, an adult (12+) one-day ticket is only 7.50 euros more than a kid's ticket.
Hopefully you'll be there on a nice day when they stay open later. (Their website says they'll close earliest at 6:00 pm but they'll often stay open until 7:00 or 8:00 if the weather is good and there are enough people in the park).
My favorite rides: The Blue Fire, the Euro Mir and the Voletarium.