I need a set shift due to childcare: ask the expert

I am due to return to work from maternity leave in June. My position is 16 hours per week on a 7-day shift pattern (different days each week). I have made a formal flexible working request for set days/shifts (for wk days only) to enable me to get childcare for my son, but this has been declined. I am currently waiting for a meeting to be arranged to discuss this. As it stands I cannot return to work without set days as my son needs to go to a childminder/nursery and because I don’t know which days i’ll be working from one week to the next they cannot accommodate him. So I’ve asked if I could only work the late shifts on week days (weekends are not a problem as my husband is at home) then my mother could look after him and then I wouldn’t need set days. But this was also declined, even though my work colleagues were more than happy for me to work the late shifts only. Please help.