If you can with your dates, avoid weekends + the last 2 weeks of December (particularly the last week) - if you can go during the week at the start of the month it should relatively quiet. I think those dates are also cheaper if you buy the tickets online (you get the MINI price, 49 EUR per adult and 42 EUR per child for 1 day / 1 park), then you simply print the tickets at home (and avoid queuing to buy them in the park gates and pay the full price)