Finally finished the romper for my granddaughter’s 1st birthday 
however, I need advice on attaching a snap tape strip for the crotch as the pattern called for buttons & buttonholes.
Great! I hope we get to see a photo.
Assuming it’s on cloth tape you could sew the strip to the knitting using a needle and thread. It might be best if you reinforce the opposite side of the tape with a ribbon in a color matching the yarn color. This way the knitting would be sandwiched between the tape and the ribbon.