It's very unusual for an employer to consider a request before you've even gone on maternity leave, so you are very fortunate. Most people go on maternity leave without starting negotiations for their hours when they come back.
You say that if you'd applied earlier, a 6 week trial would have been ample to prove it will work. Surely the same still applies if it takes place in April? If 6 weeks is ample then 6 weeks is ample, regardless of when it is.
With a trial period, it's just that, a period to trial the request and see if it works. At the end of the period you'd have a discussion with your employer to work out whether it is going to be finalised or not. You're not losing anything by doing the trial period, it's a good thing for you, and makes it much more likely you'll get what you want.
If it comes to the end of the trial period and they don't agree it, you can resign if you want to. If they do, then great.
If you resign after you are 25 weeks pregnant, then you will still get your SMP, so that's fine.
Reducing by 4 hours a week is not much at all, as you say. It does concern me a bit that you say there is no way, physically or emotionally you could carry on doing 37 hours, but you seem to feel sure that doing 33 hours with two children including a 3 month old baby will be fine. If you're already struggling with the hours you are doing, when you have a tiny baby to contend with as well, is such a small reduction really going to be doable?