I have a lot of sympathy for you OP. Your situation is very unusual because you had a flexible working arrangement which suited the employer and now they've decided it doesn't suit them and are refusing to even consider your reasons. There is a cost to you in petrol, since presumably you claimed mileage for site visits, but now driving to the office you can't as it is your ordinary place of work.
Are you absolutely certain there is no chance of a return to site visits in future? Maybe you should start campaigning HARD for that, listing reasons why the visits are more productive than the Teams calls and getting some of the site managers to agree.... unless the people at the sites are now all wfh?!!! A return to business as usual would suit you and you might have some luck with that.
Are your current 7-5 hours contractual or is that a flexible working arrangement?
If you cannot successfully campaign to do site visits and wfh again like before then you have a right to request flexible working once each year, so I would formally make that request first and propose, you come to the office ONE day a week out of four, leaving early at 3pm to collect your DD from nursery on that day avoiding risk of heavy traffic and making up the missing two hours in the evening. Explain that working in the office is a new practice as you didnt do this before mat leave, and that working 7am to 5pm in the office is difficult as you will be leaving home at 6am and not getting home with your daughter from childcare until after 6pm.
When they reject your flexible working request they have to give you a valid business reason. I really CANNOT fathom a reason that is valid. They can't say on one hand "oh great, now Teams is so brilliant we can do all our site meetings online productively" and then refuse your request saying "Teams is useless we can't get anything done unless we are all F2F in the office and you are being managed directly" - that's rubbish as you were being managed remotely not seeing your colleagues successfully before Mat leave. The inconsistency leaves your Employer open to a legal challenge. So go through the right channel and get them to put that feeble reasoning and rejection of flexible working request in writing.
Then, email your anager summarising the conversation you've had, eg 'Further to our conversation about a change to my working practices and your rejection of my flexible working request, you have asked me to commence office working on (date). I have always worked from home to enable site visits to occur, but as this is no longer necessary and you are unwilling for my wfh arrangement to continue, sadly I find myself in the position of being unable to continue working my current contractual hours due to this being incompatible with my family life. As you know I have a young child and wfh has helped me avoid a long and uncertain commute, in order to help me manage childcare pickup and drop off. I am disappointed that family-friendly working practises at (employer) don't support me to continue working from home. This seems out of step with employers in general post-covid, and I feel it is likely to deter talent from joining our organisation and encourage committed employees like myself to seek work at employers who have more progressive, family-friendly working policies. I have always worked consistently well and productively from home. I have adjusted well to the changes imposed by covid, enabling work previously done in F2F meetings to be accomplished efficiently in Teams meeting whilst wfh. I have had no negative feedback about my productivity or the quality of my work since I returned from maternity leave. However as it appears from my conversation with you there is no room to compromise on the changes to my work location, for example with me one day a week in the office instead of four, I find I am obliged to formally request a reduction in my hours of (x) per day, to accommodate the new working practices. I also submit a further flexible working request to have a shortened lunch break of x minutes so that I can leave the office earlier at the end of the day and minimise impact to my family life from these changes. Please can you confirm you approve these changes and arrange with HR the date this will take effect, and provide me an updated contract detailing my new hours and pay."
Make sure you copy in someone senior in HR and consider copying your manager's manager depending on the relationships there.
I would then work to rule, doing exactly what is asked but not a thing more. And I would hunt for another job, because honestly you are right OP most employers are bending over backwards to HELP employees wfh now. Better for the environment if we aren't all driving around, smaller office premises required, happier employees, higher productivity as employees not driving around to different sites during the working day.
And I would consult an employment lawyer and see if there is a case for indirect discrimination and constructive dismissal. Your commute, whilst not outrageous, is enough to make working 7am to 5pm unviable for most families, as it extends your reliance on (and therefore cost of) childcare, puts you an hour away from nursery if there's an emergency pick up needed (eg child sick), makes your commute more costly (2 hours driving per day), and is completely pointless as there has been no reason given why wfh isn't still acceptable. Plus you've PROVEN you can wfh successfully pre covid.
I'm really sorry also you had so many unhelpful replies here, hope you are still reading what other posts are saying as not everyone is horrible!