5 days work - whom should I inform?

I receive carer's allowance for my younger son and income support. I haven't worked for a few years.

However... next week I am working for the local council opening and sorting postal votes. I will be working 5 days (a few hours each day).

I will probably make around £100 overall.

Does anyone know what I should do about this? Is there a phone number or is there a limit I can earn before DWP would be interested?

Thanks.