Vintage YA Mystery - Papers Hidden In a Trunk... Abandoned House?

i've been trying to recall what this book is for YEARS, and now i'm turning to the internet... it was an old book when i had it, and that was in the 80s. i'm guessing it's from the 60s or thereabouts. all i remember is there were two kids who inherited a trunk (maybe their aunt's?) and they were trying to find out what she wanted to tell them by getting it open. they had a clue saying there was a "key under the frog," but they couldn't find the frog... then one of them remembered that this weird-looking iron thing was actually called a GARDEN FROG. that was a big deal - the key was underneath it. when they opened the trunk they saw it was lined with flowered wallpaper and then noticed there was a single frog among the flowers. when they slit it open they found a bunch of hidden papers. this somehow led them to a house that seemed abandoned and had a stained glass window... that's all i remember. would so appreciate any help!!!

Maybe a book from Peggy Parish's Liza, Bill, and Jed series. which starts with Key to the Treasure?

Could it be this? Search results.

The Mystery of the Gingerbread House
by Wylly Folk St. John - 1977 - 207 pages - Snippet view
"All I could make out was something that sounded like 'key— trunk— look under the frog—' It doesn't make sense, of course. We didn't have any frog statuettes or anything like that." "Did Joy have a toy frog?" Greg asked. ...

Cover

OMG - I think it's the "Mystery of the Gingerbread House!" I've done so many searches and come up with nothing - I can't believe you found it. Thanks SO MUCH!! Just ordered a copy... can wait to see if it is, indeed, the same book. ;)

:) I'm glad it was so easy to find! You gave excellent clues (year range, keywords) and they were available on books.google.com. Enjoy rereading!