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.
Music royalty for parents AND smoking hot nannies? Blue Ivy wins again.