They absolutely could do that, which is the point. UBI gives you the freedom to make the best decision regarding how to use your time. If UBI is enough to cover

They absolutely could do that, which is the point. UBI gives you the freedom to make the best decision regarding how to use your time. If UBI is enough to cover you, then spending time at home raising kids and taking care of your family may be the best use of your time, which is a win for both your family and society.