I fly 60,000 miles a year as pax. I've never experienced unruly kids on board.
Once had aisle seat with empty beside me. Young girl (maybe 2 1/2) comes down the aisle and stops. Looks at me, I smile. She then clambers onto empty seat (armrest was up), proceeds to walk over my legs to get a view out the window. My hands went around her waist to steady her. A few minutes later FA comes down the aisle looking. Gives me a smile - I gave her my help expression. Mum arrived shortly thereafter.