You need to ring up and get yourself insured to teach! Not cheap! If your a registered instructor then the BHS have a good policy but I am not sure if this is available without qualifications!
I appreciate that you may well be only teaching your neices/nephews etc but with the best will in the world if they were to fall off and seriously injure themself or a third party etc and or need long term care for the rest of their days then their parents and relatives will be looking to you and your insurance company for compensation as they will have no other choice!
Teaching children you should now also do a "protecting and safeguarding children" course and have a certificate plus you should have a basic first aid certificate.
Sorry if this sounds intense however people do sue and accidents do happen and if the worse came to the worse there is only one place they can claim from.......you!