I usually have a second grader who comes up with the negative number strategy.

How about using the open number line or the break apart strategy of bridging to a ten?

32-2=30

30-10=20

20-3=17

Because children know their tens combinations, these are easier subtraction problems.

I don’t think it is forbidden, but most students don’t learn this first, and it’s a bit more abstract. Part of my goal right now is to choose one clear approach, so people don’t feel overwhelmed.

In fact my question is actually around the notion of negative number and hence see substraction as addition of negative (also called opposite) number https://gdaymath.com/lessons/powerarea/1-4-an-aside-on-negative-numbers-piles-and-holes/.
Is it too confusing to introduce this notion first ?
What about if kids want to do 32 – 45 ? is it forbidden ?

You mean 32 – 15 = (3-1) tens and (2-5) ones = 2 tens and -3 ones = 1 tens and (10 – 3) ones = 17?

That’s a great way to do it too. Once you really understand the base 10 structure, there are lots of options.