Does your backing have a pattern you could trace on the yellow tissue sold in LQS? Can't remember the name, sorry. I like tracing an animal the child likes and quilting it all over with that pattern. I also heard someone at guild say use a sticky note in the shape you want and fallow it, outlining the notes spread over the quilt.
If you have seen my messy studio updates on this site ,you know I love thread. If I understood you say the child likes purple that is what I would do the quilting in. I try to please the person I am doing the quilt for. I don't care to much for the fishing line stuff. Yes I have 4 or 5 all different (.03mil or something) spools of it and no I don't use it often. I am not a fan of it.