Hi
@jolo289 and welcome to the forum. The people here are a friendly bunch and will help any way they can.
I'm sorry to hear your mum is in a secure unit. My husband was sectioned under Section 2 last year. I found it a very exhausting, worrying time but with hindsight was the best thing that could have happened.
If the unit is a good one and a specialist dementia unit, you will probably find that the staff will have most things under control. You will be consulted and supported throughout. I would recommend going to any meetings you are invited to if you can. This is a chance to talk to the entire team of doctors, nurses, physiotherapists, occupational therapists etc. Be open with the doctors and nurses. They are there to help you and your mum.
Things to take - the usual clothes, toiletries etc. I took favourite snacks for my husband and we would eat them in the small garden they had in the centre of the unit. Photos, momentos, scrap books that will help your mum are useful. The room my husband had was locked and only accessible by the staff so things didn't go missing. If your mum likes things like colouring, then take some books and pens. There will be activities that she can join in.
You may find the staff don't like visitors at meal times so avoid those times. This limits the amount of time you can stay. Keep an eye on your mum to see if she is getting tired, then leave quickly without saying too much. The staff will help with this by distracting her as you leave.
My main advice is keep talking to the staff. Get to know them and tell them about your mum - her likes and dislikes etc. After seven weeks I made good friends with some of the staff and one, in particular, was amazing. If your mum is able to go out, take advantage of that. You may have to have a member of staff with you while she is sectioned but they remain discreet. I found that a life-saver.