Let's line the playsuit.

Lining a garment big or small can seem confusing.  Lining is really not very hard to do, just think of it as an extra step in construction.
When I first started sewing for my little ones, I would not attempt lining anything.  Then one Christmas, my sister in law Nicki, gave me the most precious little outfit for our daughter Cami.  It was a button on the shoulder jump suit and had a LINED jacket to match.  Yes, Nicki made the outfit, she was such a beautiful seamstress.  Well, of coarse I knew I had to get over my fear of lining anything, as I wanted Cami to have more of these adorable outfits.
So, do not be afraid of lining anything!!

When lining  a garment, it is just a matter of cutting two of everything.  

Now, lets put those two, together.

 Place one playsuit back piece to front and back RIGHT SIDES together, leaving the back completely open.
You will sew the top all around together and the leg opening together, everything else stay's Open.

 This is the open back, both fashion fabric and lining fabric.  Remember you left the back open and this is why.  You will now reach in between and pull the lining out.

Line up the back fabric and the lining fabric and sew lining to lining fabric to fabric, leaving a 3" opening in your lining fabric so you can turn everything right side out.

Reach through the back crotch and pull the front crotch through to the back crotch again sew fabric to fabric lining to ling  this will form a circle and close your legs.

Now, reach in, pull the playsuit through the opening in the lining.  Hand stitch the lining closed and press well.

 There you go---one darling little play suit.