Has anyone purchased the white leotard and on the first wash it turned brown?

Yes , I bought the white leotard and NO it is still white after washing multiple times. The one I received is really good quality and fits well

What size would I get if my granddaughter where’s a size 6

I would recommend ordering size 8-10. My daughter is a clothing size 6/7, with a lean frame and the size 8-10 (140) leotard fits her perfectly with plenty of room to grow and stretch.

My 3 year old daughter wears a 4T. What size should I order?

I suggest you order the Child S (Age 4-6), unless she is smaller than average. I ordered this size for my 4 year old granddaughter and it fits her fine.

Can this garment be ironed?

The bodice could, yes, but I would hesitate on the skirt- unless using a low setting. I wash and dry every week after class and it is wrinkle free from dryer.