sit on somebody's lap = siedzieć komuś na kolanach

... sit on Santa's lap... nowadays, ...

Source: http://tz-life.blogspot.com/2008/11/sitting-on-santas-lap.html

siedzieć komuś na kolanach

English sentence and Polish translation

Come on and sit on my lap. = Chodź usiądź mi na kolanach.