I really think the carbonated juice or Koolaid is the way to have a drinkable function. I have a 5 year old who knows beer is stinky and I've thought about this a little bit. I need to get my CO2 tanks refilled so I can do the Carbonator Cap thing.
You have a few options mostly already discussed. I would take a varied approach depending on mood, and your brew day.
Maybe get a T-ball for "Steeping Grains" in a sun tea on sunny days, and IF you do a Steeped grain/extract batch. That way they can pretend to do what Daddy does.
Make a Kool-Aid Mash like discussed above when you mix up ALOT for them to drink during a long AG brew day. Find some kind of bottle that is thier "Beer Bottle", maybe use a Mr Beer Fermenter for distribution after "mashing" in the cooler. *Edit* Add some fresh berries as Grains */Edit*
I like the Carbonator Cap idea for making some drinks. It would be pretty cool for her to open her own fizzy drink.
And then for normal in the house (rainy days) I would have an assortment of "Brewing Equipment". Make a big deal out of building his "brew rig" like Daddy does! Maybe get some Catalogs and ask him to figure out what he has or is around the house that he could use. It will get his little mind owrking. My daughter has her own kitchen in the corner that she get TONS of use of things that she re-purposes into stuff Mommy+Daddy use.
Collect a bunch of re-useable bottles and let it be "Bottling Day" every now and again. There's a ton of different things you could do.
That Colored Aquarium aerator is an awesome idea for "boil"
Women are like yeast... You gotta keep them at the right temperature if you want to get anything out of them!