Was that meeting two years ago to agree a flexible working request? Not that it much matters either way. Your contract of employment, as agreed at that meeting (from what you've said) is that you will work 20 hours per week. Do you have set days and times too?
If your employer wants to change this contract they cannot do so without your consent unless there is a collective bargaining agreement in place between your employer and a Union and then you have to abide by what the Union decides, even if you are not in the Union. See here: www.direct.gov.uk/en/Employment/Employees/EmploymentContractsAndConditions/DG_10028079
What is the business reason for wanting to change or reduce hours? If your employer wants to change your employment contract (which is what they are doing), then they need to consult with you or your representative (eg Trade Union official), explain why and listen to alternative ideas.
It is worth bearing in mind that if there is a genuine business reason and your current contract cannot be accommodated it may be that the alternative is redundancy. ReduceRecycleRegift is not correct in her 3 alternatives scenario. Any alternative has to be suitable. If it is worse terms and conditions then that is not usually considered to be a suitable alternative. See here: www.direct.gov.uk/en/Employment/RedundancyAndLeavingYourJob/Redundancy/DG_10029844
From what you have said it is likely that your manager has no idea about your employment contract and what it entails. Nor does s/he understand their obligations to you regarding your contract and that comments like you being difficult because of childcare is potentially discriminatory.
What I would do now (assuming no collective bargaining agreement) is to politely point out that you have a contract to work 20 hours on X days at X times and that you have the notes of that meeting agreeing that contract. You should point out that you do not agree to the proposed changes to your contract and that should they wish to make these changes you must be properly consulted. It may be worth putting this in writing and cc'ing HR. I would also be documenting any comments from your manager or colleagues and where a comment is made by your boss point out very calmly and very politely that it is not appropriate.
I wouldn't be engaging a lawyer yet - you've not even begun the internal grievance process. Giving ACAS a ring may be worth it (but IME they're a bit hit and miss) and Working Families have a helpline on 0800 013 0313 for parents and carers.
If things have escalated or do escalate say and I'll give you some stronger wording to use.