There is no magic, sadly. Only hard work 😬
Some DCs are better at kick than others either due to their muscle tone or technique. Girls tend to be better at it than boys at that age, but there are always outliers. If she normally keeps up with her lane but drops massively behind during a kick set, make sure she is taking any coaching points directed at her on board and working to achieve them (so many DCs nod, then push off and do exactly the same as before 😏). If she isn't getting picked up on technique it might be a lack of effort, but let her coach know she's struggling with kick and can they take a look at it to see if it can be tweaked for greater efficiency.
If she isn't a strong kicker, bigger heavier fins will have the opposite effect of what you are after as she won't have the strength to get them moving quickly enough. They will also affect her body position if she doesn't yet have the core strength to hold up the extra weight, adding profile resistance and slowing her down even further.
Most of our club's younger swimmers have Maru, although these seem to snap fairly often. My youngest uses Finis Zoomers (yes, you can get them in the UK Madcats), and I've heard good things about the Arena junior fins with the heel strap (the adult ones will be too much for her from your description). I also really like Kiefer, if you can find some second hand as I don't think they are sold any more - nice supple shoe part with a stiff hard fin, and pretty light weight.
Our biggest issue has been finding a brand to fit their long but skinny feet, so we've prioritised that and once we've found a 'winner', we just stuck with it!