Planning to freelance is a good idea, many of us fall into it.
I echo what others have said.
You also need to use your time in your job to make the most of any opps that will help you once you leave. So noteworthy projects, external profiles opps like speaking and joining any networks/working groups etc that will extend your network.
You need to imagine yourself sitting with a prospective client when then time comes - what will impress them about your experience?
Then I would start signing up to your 'competitors' e-newsletters and social media channels now. You may learn lots from watching them. I watch mine closely.
If you are trying to replace a salary then as wilson says it is likely you will need to work when the opps come which includes evenings and weekends. However, it does offer flex and you have control over what you agree to.
Re money. It could be worth waiting until DC1 is in school? Depending on when DC1 is 4 and therefore starts school. That will mean less childcare expense.
You should look at what you can survive on in money terms (taking inton account DPs income, if he has one) and work out the min you need to earn and maybe save up a few months worth.
Your DC2 will be able to have the 30 hours of free childcare when he/she turns 3 (Gov are bringing it in next yr) so it is worth looking at Ofsted reg childcare, as only those settings can access the funding I believe.
In my experience (marketing consultant) it generally takes 3 months to turn an enquiry into a project. It might be a faster turnaround in HR (if that is your field) but be prepared for that.
I would also consider having a part-time role. In my first 3 years I always had a retained (related) role on the go. I liked the fact I had assured income and it meant I fitted project work around it, so was busy the whole of my working week from the go which I prefered. I have 'honed' my offer during that time too.
There is some element of when you start not really knowing what clients will want
Although if you have planned then it might be clearer.
I would also suggest sticking to what you know. It is much easier to sell yourself on your previous experience. I dabbled with the idea of supporting SMEs but have come back to my niche which is where my career was.
I dropped my last retained role in Dec as I was so busy and it has worked out ok.
However, many ppl just jump straight into freelancing.
It is completely possible to do and planning will ensure you make the best go it.
HTH
Good luck.