We have 3 cats and 3 dogs. The only way I've been able to keep the litter boxes safe from the pups but accessible to the cats is with baby gates. The main litter box is in my kids' bathroom. Fortunately they are both tall and just step over the gate. It's not unusual to have all 3 pups lined up in front of the bathroom trying to figure out how to get in (or to just bug the cats).
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__________________  Shannon & Puff 
"All his life he tried to be a good person. Many times, however, he failed; for after all he was only human. He wasn't a dog." Charles M. Schultz |