Help with opening up the back of a dress?

I made my daughter a dress using the Spring Ruffle Top pattern.  It would fit her if we could get her arms through it! Haha!  I need to open up the back of the dress a bit.  I am thinking that if I cut a little ways down the middle back of the dress then maybe put some bias tape there... or something... that this may make the dress wearable.  I am wondering about whether to put a button and loop in the back or just ribbon ties. 

Anyway, can anyone tell me how I open up the back of a dress?  I have not done this before and don't want to wreck the dress because it turned out so pretty!!!

Thanks in advance for your help!

What about making it like an apron top?  You could finish the edges and make it tie in back and she could wear it over pants or a skirt?  I have 4 year old twins and they love wearing stuff like that.

I might do that if I get stuck. Thanks for the suggestion!

 I think I would rather just open up the back a bit near the top though.  Is it possible to just cut down a bit, somehow finish that edge and add a ribbon or button?

Maybe open it under the armpits and sneak in a little extra fabric or elastic there?  Might be easier than trying to finish the back.  But there are a lot of talented ladies here that might be able to guide you.