Windows – stand-alone* wildcard for the user documents folder

dir *.dll /S
del *.dll /S

If you really want to be careful:

del *.dll /S /P

Why bother with a UI?

BATCH FILE TO DELETE FROM SUBFOLDERS WITH DIFFERENT NAMES DYNAMICALLY USING WILDCARD FOR SUBFOLDER NAMES

Batch file to delete files from wildcard folder

Here's an easy and simple batch script example that I use to complete this type of task all the time, I plugged in the variable folder paths to suit your needs as you describe:

SAMPLE BATCH SCRIPT

(Set your variable root folder and subfolder up top, and the FOR /D and FOR loops iterate accordingly to do the rest of the magic traversing the directory as the logic specifies and completes the DEL /Q /F command for the *.txt files)

@ECHO ON

SET SourceDir=D:\L1
SET SourceSubDir=L3

:: --// note the asterisk wildcard after SourceDir in the first FOR /D loop using X as the variable
:: --// note the X variable appended to the beginning of the second FOR (no /D switch here) loop in the SET part using Y as the variable
FOR /D %%X IN ("%SourceDir%\*") DO FOR %%Y IN ("%%~X\%SourceSubDir%\*.txt") DO DEL /Q /F "%%~Y"
GOTO EOF

NOTE: If you plan to run this with a copy and paste manually in the command prompt, then the variables in the FOR loops need to have one of the percent signs removed in all parts so use the below for that part if you're running this manually with a copy and paste rather than in a batch script and executing that which is how the above example will work.

FOR /D %X IN ("%SourceDir%\*") DO FOR %Y IN ("%~X\%SourceSubDir%\*.txt") DO DEL /Q /F "%~Y"

FURTHER DETAIL AND RESEACH

(Type in FOR /? in Windows Command Prompt to see this detail)

FOR (with no switch)

Runs a specified command for each file in a set of files.

FOR %variable IN (set) DO command [command-parameters]

  %variable  Specifies a single letter replaceable parameter.
  (set)      Specifies a set of one or more files.  Wildcards may be used.
  command    Specifies the command to carry out for each file.
  command-parameters
             Specifies parameters or switches for the specified command.

To use the FOR command in a batch program, specify %%variable instead
of %variable.  Variable names are case sensitive, so %i is different
from %I.

If Command Extensions are enabled, the following additional
forms of the FOR command are supported:

FOR /D

FOR /D %variable IN (set) DO command [command-parameters]

    If set contains wildcards, then specifies to match against directory
    names instead of file names.