Changing the definition of full-time hours to 30/week would do far more for families and children than giving free childcare so mothers can work more.
Making mortgages with a > 20 year term illegal, putting limits on the total principal allowed to loan as a multiple of income, and barring entirely non-human (i.e. any business entity) ownership of single family homes would do far more for families and children by removing the burden of ridiculous housing costs by removing the ability for people to compete for ridiculous housing prices.
You want to change what now? The dictionary definition does not specify any particular time. There is no legal definition for full-time. The IRS uses the term full-time, but they actually use it exactly like you wish: 30 hours per week.
People out on the street often casually use full-time to refer to 40 hours per week. I anticipate that is what you are referring to. But that usage is simply used to refer to how many hours they are working. 40 hours under that usage is an observation, not a commandment.