Sometimes a vanity just doesn't suit the space and those using it. If swapping it out is not an option, Diana Burton recommends a couple of workarounds, particularly ideal in children's bathrooms where they might not be tall enough (yet).
'For a too-high vanity in a kid’s bathroom, a simple step stool works well.' You can find aesthetically pleasing footstools with curved designs, like this Eden Creations small wooden stool on Amazon, which can double up as a plant stand when not in use.
If you are fine with a little DIY, you could edit the countertop itself: 'For a too-low vanity, consider a thicker mitered countertop to add 2-3 inches in height. Alternatively, raising the vanity on a platform that matches the flooring can achieve the desired height, though it may increase labor costs due to plumbing adjustments.'