The TUC said the figures for England and Wales showed the work-life and childcare challenge facing families.
The union organisation said some parents of young children found it difficult to fit their shifts and working patterns into school opening times.
Bosses should listen seriously to requests to work flexibly or take unpaid parental leave, it was suggested.
TUC general secretary Frances O'Grady said: "The beginning of September marks a huge milestone for children starting reception class, but for many working mums this time of year can also be a logistical nightmare.
"It's important that starting school is not rushed and that children take their time to become settled and confident in school.
"Some families will really struggle to juggle childcare arrangements alongside work, and bosses need to be sympathetic to flexible working, work from home and leave requests.
"Flexible working has been a roaring success for both staff and smart employers over the last decade, despite grumblings from some business lobbyists."
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