Boundless Magic by T.M. Cromer

Autumn and Keaton Carlyle lead a charmed life. That is until their young children vanish one fateful day, plunging their world into turmoil. It isn’t long before they discover the combined magic of Chloe and Jolyon has accidentally opened a gateway to an alternate reality, drawing the kids into its fantastical depths.
With the help of trusted family members, Autumn and Keaton set out to find the spell to reopen the portal and retrieve their children. But it isn’t until their team faces their alternate-reality clones that they realize it’s Opposite Day in this bizarre new realm, and locating Chloe and Jolyon requires the help of a back-country Alastair along with a Gothic, surly Spring.

Now, Autumn and Keaton must make nice with their other-world counterparts if they stand a snowball’s chance of finding their lost children and returning home.