Hi!

You could make the seashell out of large pieces of carboard. (Some cut up large boxes would be ideal). If you make it out of lots of pieces you will be able to get the curve. Tape the edges together with masking tape.

You will need a strong base to take the weight of where your daughter will sit.  A piece of wood maybe. You could then build up the shell from it to look decorative.

There are many different adhesives you can use. Visit here for the basics:
http://www.papiermache.co.uk/exec/cms-d … iew/id-23/
If this is likely to be a one-off project for you, I'd recommend wallpaper paste.

Good luck.