I have a 2016 Renault Captur. My daughter dropped a small plastic toy (about an inch high) in the windshield vent (the one above the dashboard) and I'm unable to fish it out. How likely is this to cause problems? I guess the worst case scenario would be that the heat causes it to catch fire, if it went deep enough...
This really shouldn't be an issue, unless your daughter is missing the toy ;-) The toy is most likely just sitting some place in the ducting, never to be seen again. As long as it isn't a priceless heirloom, I don't think I'd worry too much about it. If you don't hear it rattling, even more of a plus.
As for it catching fire, the HVAC system in a vehicle will never get hot enough catch fire. You have to remember, a car's coolant system runs just a bit over the boiling point (if not lower). This might be hot enough to burn your hand or cause you bodily harm, but it would never be enough to catch a toy on fire.
I used to work as a design/ quality/ manufacturing engineer for an automotive company. I once worked on a car that the owner complained had a premature airbag deployment. Myself and a couple of safety engineers reviewed the car at great length.
The air bags had not been deployed at all.
Turns out the owner stored a bunch of stuff on the top of his dashboard. A butane cigarette lighter had fallen down the defroster grill. When the owner was driving (in the winter with full heat in defrost mode) and the lighter burst open with a violent bang. No way did the gas ignite, but the loud bang sure surprised the driver.
We were able to retract all the shards of plastic butane lighter to share with the owner.
The highest temperature I'd expect to see inside the defroster would be quite a few degrees less than the boiling point of water @ 13psi (somewhere around 240 degrees F). Obviously less than that maximum because you are mixing in outside air. And most folks use their defroster at full heat only when it's cold outside. Don't forget those ducts are made of plastic, generally polypropylene with a talc additive.
Simple plastic toys should not be a problem. Butane lighter? Pressurized CO2 cartridges? Batteries? Not a good idea.
One thing: here's a great addition to your mechanics tool box. These come in way handy for a whole bunch of tasks. The even make em with a tiny led bulb at the grabber end.
You squeeze the top knob to extend and open up the jaws of the grabber. When you let go the jaws retract and grab stuff securely.
Plastic typically doesn't melt until very hot (325F for some of the lowest ones). For it to catch fire would be even higher.
The risk of catching fire are very minimal considering automotive heaters do not get anywhere near that range. Typical vehicle heaters are in the range of 115F to 120F.
The largest risk, IMO, is the item being forced out of the duct by forced air while you are driving.