If you come back after ordinary maternity leave you must be given your old job back. If you come back after additional maternity leave your employer doesn't have to give you your old job back if it's not practicable to do so, they must instead offer you a suitable alternative.
Sometimes this takes a bit of time to sort out, but not 8 months, that's not acceptable. What reasons have you been given for not going back to your old job, and for the lack of a defined role? When meeting with your boss, what does he/she say about what the problem is?
I don't think it's a question of 'reviewing' it. It's a question of sorting it out. You need to be much firmer with your boss than you can have been. Get him/her to put a date in the diary for a review meeting at which you expect to receive clarification of what your role is going forward as you do not consider what you are doing now to be suitable alternative work to your original job. Are HR involved? If not, why not? Copy them in.
What reason is your boss giving you for treating you differently when it comes to time off for children being ill?
If I'm honest I think you are being walked all over, and I think you need to examine your own behaviour to work out why this is happening. If it's been 8 months and nothing's happening, your boss obviously feels treating you like this is not a problem, you won't make too much fuss or cause any difficulties for him/her. By this stage you certainly should have done.
Get stronger and more firm. Insist on action and agree a timescale for it. If it doesn't happen by the timescale you agreed, put in a grievance immediately. Demand fair and equal treatment when it comes to time off and do something about it if you don't get it. This includes requesting holiday time back where you have had to take it and others would not have had to do so.
Sorry to be a bit harsh, but this has gone on waaaaay too long, and you have to take some responsibility and action yourself to get it sorted. Get help and advice from HR and be a lot more insistent and formal in your approach.
Hope things get better quickly.