Toilet splash block

I've got a ca 1997 Toto low flush toilet with a problem. The splash block/water level in the bowl is low, leading to excessive soiling of the porcelain. Worked fine up until a year ago, when I had to plunge it repeatedly for about a month. The problem wasn't line blockage, it was a 4 year old who was so pleased to have learned the skill of wiping he, erm, overdid it. ;-)
His lordship also practiced his skills on the upstairs toilet, a ca 2000 American Standard Cadet, with the same results (pretty much lived with a plunger in my hand for a couple of weeks . . .)
The Cadet's splash block/water level is fine but the Toto's isn't. Any ideas about how to restore the water level to a near normal? Right now it's about 2" above the throat of the flushway, providing a splash block about 6"x6", which is entirely inadequate.
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