I went through this process of reducing my hours before I finally retired on medical grounds.
To add to what Carole has written, your employer has to conform to employment legislation and make ‘reasonable adjustments’ to your working conditions and environment, to enable you to carry on working.
You can request a workplace assessment by an Occupation Health assessor who will report back to your employer and you can all then agree to what is required for you to continue working. I do not know whether your employer is a large or small firm, but larger organisations will have their own OH and Human Resources departments with experience in these matters.
As Carole says, it is essential to contact your union rep, assuming you are in one. The rep can attend all meetings with you and be instrumental in negotiating the best arrangement for you. In the initial stages, the rep can advise you with the wording.
Don’t forget that you are the important person here and you must negotiate the hours and conditions that work for you. It is easy to overestimate what you can do and then find later that you need to revisit your hours, especially as you have a child.
I hope it works out for you. Congratulations, by the way.