I have a 2 year old that used to do the same thing, but not anymore. When she was less than a year old, I used 2 pads to give her more room, and she still miss or would be half-way there. Then I got a tip from one of the threads in this site, to lay down a small half-filled water bottle with it's label peeled off on all three sides next to the pads. To my surprise, it worked. I kept the bottles for about a month, and eventually took it off. She still stayed within the pad since then. Just remember not to get the pads too soiled, as they will try to find another cleaner to do their business. Hope this works for you.
Mochi, Skittles and Twinky's MOM |