There is no specific rule against it, however the protocols for any arrangement should be discussed, such as the information in the Code of Conduct which says:
On occasions, staff may undertake babysitting and home tutoring outside of school time. These are separate arrangements and are undertaken by staff in their private capacity and not as representatives of the school and, therefore, no liability will rest with the school as a result. It is the responsibility of staff to inform the school that they are doing such babysitting or tutoring duties so that the amount of hours being worked per week can be monitored to ensure compliance with the Working Time Regulations. Members of staff will be responsible for accounting to HMRC for any income received from such work.
And also confidentiality, safeguarding precautions (so ensuring they a not in the premises alone with the child) and ensuring there is no favouritism.