It would be eaiser to manually do it.
Your probably going to key the visibility anyway using the MEL script.
You would have to initially set the bats visibility to 0 (either using a varaible or setting it in the attribute editor then either set a variable to be the same as the time, then use an if statement to say when the time value of the time gets to 10 to set the visibility of the object to 1.
Its a simple script thats pretty easy to knock together, but you would be best off just keying the values.
"No pressure, no diamonds" Thomas Carlyle