Have a look at what other childminders (and nurseries) charge in your area... enter your postcode here
Also let us know on here your rough location, as there may be childminders on here near you, who can give you more local info.
A concern I would have is how many children you would be permitted to care for - given that you would have 2 under 5's yourself, plus a 5 year old. Have you considered how many children you would have to care for, to make a reasonable income? I expect Ofsted (regulator in England) will permit you to only care for 1 minded child under 5 plus 2 children aged 5-8 years at a maximum. You won't be limited as such for over 8's, but you may not want many, if any at all, at least until you have more experience of caring for other people's children.
In my area (West Surrey/East Berkshire), £5 per hour seems quite typical for childminders.
For before/after school care - consider having a minimum charge, as some parents may only want an hour.
There are lots of message threads on here regarding childminding, including comments from parents who use childminders and childminders themselves. It can be interesting reading, as you will find out about some of the common issues that arise.