In that case you will need to register as a childminder - contact your local authority. The process can take many months. When your friends child is 8, then you don't have to be registered - unless friend is using tax credits or childcare vouchers. So may be worth waiting until your friends child is aged 8.
It can cost you as much as £800 to register. It may not be worth it unless you want to look after other children too. Find out if the friend wants to use childcare vouchers or needs help of tax credits to pay for childcare. If she doesn't need any financial help you can skip registration and have her child after school for payment but no longer than two hours at a time. Holiday care is more tricky ... you could do it for free and ask her to reimburse/provide you with food for the child and any entrance fees or work from her home. If she wants to claim help with childcare cost you must be registered.
You would be very vulnerable without insurance, and of course insurers will want to see you are registered and all legal and above board. You would also need to register as self employed for your part time minding job, and have the hassle of completing your own tax return and paying your tax as presumably your teaching job will use up your personal allowance. Then you can't pay the tax if you are not working legally, so I would have thought the risk of being reported for either unregistered childminding or tax evasion would be quite high - it only takes a falling out with the friend or an argument with a disgruntled neighbour. Would that not have serious fallout for your teaching career?