Lottie does DC(8) enjoy baths anyway? Many find the water soothing and it gives some kind of proprioceptive pressure that helps too (or something like that - I pick up bits at each course I go on, and some stick better than others).
We have a DD (about to turn 12) and when things are very tough, she will have a long bath and play with the toys for maybe an hour. Comes out very wrinkly but sooooooo much calmer. So the "playdoh" type logs of bath bubbles (come in a few colours) from Lush, or the "Icklebots" bath bombs filled with lavender, or "Intergalactic" which changes colour as it dissolves into the water are firm favourites. They've just introduced a "fidget spinner" for bubble baths, which actually spins (not well) too. I know there are other companies out there, but the lush ones really seem to have good smells and colours and make it fun, so DD will go for the bath.
And we replace the toy ducks and squirters every couple of years - I know she's old to have them, but she really does play with them still and it extends the time in the water which all helps the relaxation process. And she still loves her original stacking cups set too (they can get washed in the dishwasher - but the ducks and squirters get stagnant water inside so need replacing). If your DC enjoys the water, things like bath toys for small DCs (we buy them in JoJoMamamBebe and Tiger has some good ducks at times) or bath crayons are great.
And we use a lot of lavender shower gel to get her clean and relaxed at the same time - I don't know if J&J baby bath/gel is still available, but both Source and M&S versions are good.
On the other hand, a shower is great for energizing her in the mornings as the water pounding down seems to act differently (not all DCs can handle the shower though, as it can feel like too much on their skin - while she loves it, DD can only handle the regular shower, and not a power shower as we've had in some hotels).
Sorry for going slightly off-topic....