Bear in mind that the £100 per week is after the following:
"Expenses that are allowed are for things like:
-some National Insurance (NI) contributions
-Income Tax
-half of any money you pay towards personal or occupational pension schemes
-other expenses you have to pay because they are a necessary part of your job
After allowing for these things, you are allowed up to half the rest of the money you earn to help meet the cost of paying someone else (but not a close relative) to look after a child or children, or the person you look after, while you are at work.
Please note, when looking at your income, Carers Allowance do not count Occupational or Private Pensions as earnings." Directgov
So actually, you could be earning substantially more than £100 per week before you actually lose your carers' allowance.
The other thing to bear in mind is whether you are working 'term-time only', ie. you get paid during term time but not during school holidays, in which case your pay will fluctuate; or if you are working 'term-time only' but they are averaging your 39 weeks of pay over 52 weeks.
If there is a recognisable pattern then you could argue for your pay to be averaged over the 52 weeks, which will reduce your weekly pay.
One more thing to look at is whether you are 'freelance' as in self-employed or whether you are an employee.
If you have to do the work specified in the time specified for your specific employer, using their equipment, then you should be classed as 'employed'. If, however, you choose your hours and who you work for, etc., then you are self-employed. You need to be very clear, because the rules differ.