Foxes living under deck.
Foxes are not uncommon under decks or sheds, and are comfortable living in the suburbs or urban areas. They shy from humans, so they will not be approaching children. Here is info about foxes: https://extension.umd.edu/hgic/topics/foxes
Foxes are predators of mice (which spread Lyme disease), so they are beneficial to homeowners. Think about what may have attracted the foxes to your yard. If there are bird feeders with spilled seed that attracts rodents, for instance, that may be attracting the foxes. Avoid feeding the birds in the spring to fall months, in that case.
We'd recommend that you wait until the baby foxes (kits) are 9 weeks, and then take measures to encourage them to evacuate their den. This page has many good ideas for how to do so: https://www.humanesociety.org/resources/what-do-about-foxes
Meanwhile, romping kits can put on quite a show, like puppies, for the enjoyment of you and your grandchildren.