Hi Matt -
GM gave good advice, depending on which thinner you spilled will determine how recoverable the canopy is by polishing. If enamel then a fair chance, if lacquer thinner which is much more agressive then less chance. By using progressively finer polishing pads and plastic polish, you might be able to restore it to usable.
And his suggestion to paint it black has worked for me several times. Welcome to the forums, great idea to involve your son, some fun times to be had modeling together.
Patrick