For example, if you lose too many benefits when you get a job, it can easily make getting a job yield negative expected value, this is bad because often it stunts future career potential.
There may be families that cannot quite afford to be a stay-at-home mom even though they want to. Providing the waiver also increases the overall fairness. In rural areas there are generally far fewer childcare options, so this becomes a benefit that accrues to those that live in cities. Not very fair.
Government services exist to help people who need them. The idea that government services need to have the same net effect on every citizen is unusually popular in the US and is part of the reason we have worse government services than our peer nations.
The reason we have worse government services is because there's no attempt to make them fair, the benefits are almost always highly skewed along partisan lines and thus usually not passed.
The same is true for things like childcare and education. Improving outcomes for the next generation doesn't only benefit them and their parents, it improves the entire society.
>The reason we have worse government services is because there's no attempt to make them fair, the benefits are almost always highly skewed along partisan lines and thus usually not passed.
You're just debating whether "everyone gets the same" is a better definition of "fair" than "everyone gets what they need". The only way for the government to satisfy the former without UBI (which I would support) is for the government to offer extremely limited services. That's the situation we're in. Because as I have said in another comment, the same argument that applies to stay at home parents applies to childless people so offering any childcare support is unfair according to the "everyone gets the same" definition.
If you live in a city, there's a good chance your house hasn't been on fire because of the work of the fire department.
I think it's worth considering what has significant majority support. For example I believe it's something like 80%+ support some kind of childcare subsidy or tax credit. Some childless probably make up the 20% just as some would prefer not to have a fire brigade.
At that level of support just pass the subsidy / tax credit and let the families figure out how to apply it (paid daycare or homecare).