Michelle Williams -- Blue Ivy Has ANOTHER Famous Babysitter

Michelle Williams wants to make one thing clear -- babysitting for Beyonce's baby girl is just a part-time gig ... because she's not the only Destiny's Child singer doing diaper duty for Blue Ivy.

Last night in NYC, Michelle revealed Kelly Rowland also pulled a shift watching B.I. in Beyonce's hotel suite ... during B's concert in Atlantic City last weekend.

In the middle of the show, Beyonce thanked Michelle for babysitting -- but Michelle 'fessed up to us she had some help ... and even got to go downstairs to see some of the show.