Have a lot of experience of small children on my kiddyback. The thing about stoker kits is that the child sits a lot higher up. For handling there will be a small effect, but the biggest issue is climbing on and off!
You will also need an extending stoker stem to bring the bars much closer.
Toeclips have always been a must,in a small size. It keeps their feet planted and that’s important when there is no independent coasting.
I added a child-specific saddle and crank shorteners too.
One complication is that crank adaptors normally bolt to the seattube. They were historically for steel frames and hence small diameters. Cannondales, whilst lovely bikes, use oversize aluminium and either the adaptor may not fit, or I’d be wary of crushing the tube.
For a five yo girl, you would see better value sourcing a proper used kiddyback: either a Thorn or (rare as ever) a Dawes Toucan. It will last you eight years and you will easily pass it on. The Tandem Club wanted pages is a good start.
But buy now! I’ve ridden 1000s of miles with small children (see some previous posts) and it provides by far and away the most smiles/mile of any bike I have ever owned. I can’t recommend them enough. Tag alongs do not come close.