Every shower I've been to (as opposed to any other party) has been child-free. Primarily for the reason that a shower is x number of adults sitting around opening presents and passing them around. UBER boring for kids.
If you are going to have kids... setting up a room with a TV, movies, snacks etc will cut down on the tantrums.
Showers aren't like weddings where kids only have to sit for 15-30 minutes tops and then there's dancing, etc. Showers are typically 2 hours + of sitting and oooohing, ahing. Imagine a kid's birthday party. While the birthday child is opening presents you've got the rest of the kid's attention for MAYBE 15 minutes... and most want to "help". Then they scatter because they're jumping out of their skin. Transpose that to 2+ hours. And presents that are (by and large) boring or toys they're not allowed to play with. Disaster in the making.