How to use possessive apostrophe with words in quotation marks?

How to us possessive apostrophe with words in quotes? For example,

...a few days later I discovered that those five little boys were not that well-behaved (as I firstly thought). In fact they were quite mischievous and conniving. The broken tree next to my house and the cat with its legs tied were all their "inventions". And Lilly, as it later turned out, was able to get to Denver so fast not without those "little inventors'" help.

You've used it correctly there, but you should consider using the more specialized punctuation marks ', “, and ” to provide some distinction:

And Lilly, as it later turned out, was able to get to Denver so fast not without those “little inventors'” help.

Alternatively, you can avoid the whole problem by using of:

And Lilly, as it later turned out, was able to get to Denver so fast not without the help of those “little inventors”.