It sounds like a good plan. I had a baby in the middle of my PGCE, so I did the first school placement whilst pregnant, then had baby, took 9 months off, then did the second placement after I'd had baby. No maternity pay though, because I was studying, not working. It was also a flexible PGCE, so it allowed for this. I wouldn't worry about when to have 2nd baby, just start trying whenever you want, don't worry about it's impact on your teaching career - you can always work around it. If you try to fit it in with teaching, you'll spend every year thinking "it's not the right time" until eventually you run out of time. In fact, in your position, I'd get on with having another one now.
If you're going to do it, then do it before your children start doing loads of after school stuff (usually around age 7) - it was at that point that I found the whole teaching thing a nightmare and I had to go part-time.
When they're still in nursery, you can drop them off at 7.30, be in work for 8am, work until 5.30pm, pick up at 6pm. They've had tea, so just a case of bedtime routine, into bed at 7pm, then time to do extra work in the evening if needs be. I worked on a weekend as well, with my husband and I splitting the day up, so eg. I would have DD in a morning, then work in the afternoon while DH looked after her, then we'd do a similar thing on Sunday as well, so I got 2 afternoons to get work done most weekends.
You can't do this though with older children - my older DD seemed to want to try out every extra-curricular activity going, so I was ferrying her around to various activities until about 7 or 8pm (it gets later with age), then she'd not go to bed until about 8.30, so 9pm was the earliest I could get started on work, and I wasn't in the right frame of mind to start work then. AND I had a toddler to drag around and keep occupied on top of that, so it wasn't even like I could get work done whilst spectating at her various clubs.
On the subject of experience, I don't know what they expect, because I'd worked as a TA for a term in the UK, and worked abroad for 2 years as an unqualified teacher, and they were very happy with that level of experience.
I suspect the need for experience is because you really are left to sink or swim and so many PGCE students sink rather than swim, but if you get loads of experience, then you'll either be put off before you start the PGCE or you'll have learnt loads from your experiences and will swim rather than sink during the PGCE.