Milk is best to be made in water of approx 40 deg to not kill the probiotics and some other nutrients
Milk is supposed to be drunk freshly made
Even if you dont't drink it immediately, it can only stand at most 1hr in a warmer.
To avoid diarrhea, you should make it fresh. You save on the trouble now, it will give you more trouble later at the expense of your child
Is it worth it to save that effort?