I've been reading the Resorts and DVC boards every day for the past 16 years. In all that time, I have NEVER heard of a guest of a Disney resort being made to stay in a room that has been found to have bedbugs, much less being made to stay in a room after it's been treated with pesticides. Not one single report.
Trust me, if it ever had happened, it would have turned into a 100-page thread on the Resorts board.
And on the tiny chance that a resort manager would completely take leave of their senses and attempt to keep you in a room that had been freshly treated with pesticides, all you would have to do is say "No, that's not acceptable." There are 30 resorts on Disney property. They'll find you a room someplace. No one is going to physically force your family into a room full of bedbugs or pesticides.
I get the tendency to worry as a new parent, but save your worrying for things that might actually happen.