@SamSam786R your contract of employment requires you to be available for work on a full-time basis from 9am to 5pm, presumably with the entitlement to a one hour lunch break.
I am afraid your employer is entitled to expect you to comply with the terms of your contract. It's why the money appears in your bank account every month.
At the very beginning, your employer should have insisted on a formal flexible working arrangement.
Are you available for all required meetings, or have your pick ups meant co-ordination issues?
Have there been delays in you responding to urgent issues?
Are you bonded into your team and the business?
Have there been virtual meetings with a child in the background?
It seems as if work is not your priority I'm afraid which for your contractual hours it has to be; your childcare needs to be arranged to suit your contractual working hours. The fact that you will be worse off financially arising from your third child, and continuing to fulful your contractual obligations is your issue, not your employer's issue.
You've had a lot of licence to date, and the birds have come home to roost. I think you accept you need a few years as a SAHM or you accept that for a few years, it costs your wallet to maintain an unbroken work record, career development, progression of earnings and your pension.