Childless people are free to start businesses to provide services that can take advantage of those rebates.

The argument you’re making in general is a valid one about subsidies, it’s a weird argument to make regarding children since having children is the only way society survives. Unless your claim is that we’re overpopulated but generally people in developed countries are not reproducing, and a meaningful part of that does appear to be the cost.

So the answer is that this specific subsidy is net beneficial as we want to make it easier for people to have and raise kids, not least of which because it produces better adults when those kids grow up and makes society healthier.