At my nephew's, lots of pushing and shoving and standing on seats with video cameras!
Catholic churches are generally very easy-going at congregation level - nobody will mind if you don't know what to do, and if you don't feel like kneeling then don't. Lots of catholics don't kneel.
The children will have some kind of talk within the set format of the Mass, then they will all go up and receive communion, then there'll be a blessing and badabing, badaboom! One more sacrament completed!
Many parents make this an opportunity for status display by dressing their kids in all sorts, can be both funny and sad to behold.