I worked through pregnancy during my time as a nursery worker and moved from toddlers to our younger babies who were not mobile. When I was needed to cover with the toddlers I'd often lead table activities and be sat down. But I have been in settings where one of my team had sciatica and because we were a family, we got her a chair and cushion and she was free to sit on bad days (so we had the extra body in the room) she also took on all aspects of paper work, daily diary sheets (back when it was written) developmental records etc to help balance the workload. Speak to your manager and if they are unwilling to be accomodating take sick leave( I don't know the financial implications for you.... I never got sick pay) join a union if you haven't already.
Do you get full pay on sick leave? If you're experiencing pelvic girdle pain (PGP) and need a sick note, book an appointment with your GP or midwife to discuss your condition and request a sick note to be medically signed off work.