Sons raptor 80 transmission locked up any ideas?

My son (7 years old) and I were trail riding today, after going down a steep hill (slick, muddy, low traction) we rode 150' to a flat part of the trail and shut them off.  Walked around and talked for about 10 minutes.  Got back on, sons raptor 80 would not start.  The starter hit hard but did not crank, does not turn over at all.  I put the trans in 3rd gear and pushed it but the tires just slide.

Towed it home in neautral, pulled the starter, starter is good.  The gear that the starter engages will not turn. Weels will not turn when pushed forward, only backwards.

I can feel the 3 gears with the shifter but the trans feeel locked up tight.

Is it time to pull motor, remove side cover to see what is up?

Any Ideas?  What to look for?  

Yes that sounds about it. could be the starter gear or something got in it. if it has a vent tube something could have got in or just went on you but check for small rocks metal from the gears. maybe a pin from a sink or a sink could be bent.  
The Devil Made Me Do IT

If the piston or crank seized, wouldn't I still be able to push the wheeler foreward with it in 3rd gear?  The auto clutch would slip allowing it to roll forward right?

Thanks for any info..  

Hello,
Does the engine turn over manually by pushing it in reverse? If it is an automatic then the wheels will turn, but not the engine. If it's a manual and the motor turns over when you’re pushing it back then I would suspect your starter as they have a cam that engages the gear in only one direction. There may be an excessive amount of fuel in the cylinders that prevent the motor from turning.
 Take out your spark plugs to relieve any pressure. Liquid does not compress. Also, if you have a manual starter this also has a cam and if engaged will only allow you to move in one direction. If it ran fine to where you turned it off it seems rather strange that it could be anything vital inside the engine. Check your starter mechanism as it can just fail without using it. It may have come apart and is binding your engine. Your clutch can do the same thing. If anything severe had happened to the engine it is highly likely that it would have made a loud and obvious noise or failed while running. An engine does not break standing still.
 Another possibility is that your transmission may be engaging two gears at a time. This is caused by a faulty shifting fork or gear location control such as a detent or bearing that keeps the gear in a specific location. Also, one of your transmission shafts may have moved (shifted) slightly due to a failed shim or a nut coming off the end of the shaft. As your bike is moving the active gear is meshing with another and because of the speed differences any other gears that may be uncontrolled will not mesh or engage until the engine has stopped. Then it may just slide or move into place with the engaged one thus engaging two different gears/speeds (ratios). This is quite possible. Check the simple stuff first as it is easy and free. No need to spend money if you don't have to. If you don't find the problem then please repost and I will respond.
Good luck.