I would send a get well card unsigned, saying heard you were unwell and was thinking of you. I would also arrange for flowers to be delivered in the name of the grandchildren. This lets your mum know you are thinking of her and are aware of her situation.
If your father is controlling it is very hard to make any inroads into his way of thinking but by sending cards etc from yourself and the grandchildren you are doing something positive and showing them that they are still important to you. It is also less confrontational.