The recipe says 1 cup of flour and .75 cup of sugar for 24 cookies; but we're making 18, so how much of each do we need now?

Math is everywhere. All you need to do is show it and show how it makes getting things done easier. Show the reason we have "math" to someone and they'll want to learn it on their own.

Yes! My 5 yo daughter can count up to 12 or 19, perhaps 29 with some help. But she only groks up to 5. For example she understand that 5-2=3, but she does not understand that 6-2=4. (In a story, like "There are 5 apples on a table, and I eat 2, how many are left?") She is improving, but it's slowly than what imagined before having kids.

As the sibling comment explains, for older kids proportions is a natural and important topic, but it's too much for my daughter yet. (I recommend to start doubling or halving the recipes, and later more advanced stuff like 18/24.)

I guess it's not possible to advance too much after that with kids, but there are a few nice initial activities.