I was given a Revell Messa-Chnitzel 109 (1971 edition) unopened, still factory sealed. Sadly, while it was sitting on my desk, one of my sons decided it seemed like a fun project and proceeded to open it. When I found him, he had all the pieces seperated and was looking for glue.
With its current condition (all parts are present, decals, instructions, etc.) is there any value left?
Thank you for your time 
It was reissued several years ago, doubtful it is worth more than $20 unopened. Now probably $10.
Yes, something that was probably worth $ 30 unopened if you really tried is now worth butkis. But no great loss, might as well build it.
And don’t be too hard on your boy- seems like he’s got the modelling bug ! (A 1/35 Dora Railway Gun will keep him away from your kits for at least 8 months)
I highly doubt the kit has any real value. Limited edition run / harder to find kits do tend to have value in them. So why hold onto them when you can build it?
A model kit is only worth what someone is willing to pay for it.
Please don’t be like certain E-Bay seller’s, that put a $200 price tag on a kit that was released yesterday for $20. Their going to "sit " on their kits and likely wait year’s for a “sucker” to buy it.
Your kit has recently been re-released so the price of the older version is now worth less than it use to be because the market is now “flooded” with them.
Your best bet is to just build it and enjoy.
I agree with the “build and enjoy” sentiment. The real value of a model kit is the pleasure you get from making it come together - plus, you have a son who shares your enthusiasm. You’re a fortunate father!
Mike