Once you have put the water in, you can add anything.
Cosmacol has a lemony/limey flavour. Tropical squash works well.
It doesn't lose effectiveness if you freeze it....you can make it into lollipops
You can make jelly from it
Pop it in the fridge and get it cold and drink through a straw.
The water is doing the actual work, so you must add the 63ml to each sachet, but you can add more water to dilute the flavour a bit....but then they have to drink more volume.
If it's the lemony/limeness that she dislikes asked to be prescribed movicol paediatric plain.....it's unflavoured (still has that weird sugar/salt taste but not the artificial citrus). Then you can add other flavourings.
You can ask for the chocolate flavoured movicol. I think it looks like the seventh circle of hell, but works for some kids. Once you've made it up, you can use it on cereal.
I run a children's constipation clinic 😂 I probably have other tricks that I can't think of right now.
Good luck.... It's a bit gross but it's great medicine!