though I’m not sure I’d call it discrimination.

It most definitely is discrimination. Because discrimination is "treating two things differently according to a discriminator". Such as living alone or with a registered partner.

Now, the purpose of this is obvious (having a child is expensive and a good thing in the state's eyes). Such cases where the discrimination "makes sense" (as it evens the playing field isntead of skewing it further) is usually termed as "positive" discrimination.

But as it technically fits the core requirement of discrimination, it is discrimination.