Our older son is 13, seven years older than our milk-allergic son. We
discovered the allergy when the little guy was almost one year old. So, after
almost 8 years of the older one eating whatever, whenever; things had to
change a little.
But we did not remove milk products from the house. Sometimes, when my older
one wants macaroni and cheese (one of his faves), we make a similar pasta dish
(elbow pasta, ground beef and sauce made at home) for the younger. They both
get what they want.
When the older has a dessert that the younger one can't have, we make sure we
have something equally special (and milk-free) for the younger.
Sometimes, the older one wants the same thing (like Tofutti), which makes our
little guy very happy.
Of course, it might be just a bit more difficult because your 5-year-old might
need more supervision when eating so she doesn't expose the little one to
allergens.
But we've found this approach works well.
And our younger son knows this is just how life is....we have to be careful.
We have to read ingredient labels every single time. My older son even helps
read them (I double check) but he's good at it. And he likes watching out for
his brother.
As stressful as it is sometimes, we didn't want to make the house milk-free
because we want our younger son to learn how to live and stay safe (without
being completely paranoid?) in a milk-filled world.
So far, so good!

Good luck!
Rose