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If you have their consent to do this to them then you are free to take that action. It would not be a breach of the criminal law to tattoo a person with their consent.
What you need to watch here though though is that this does not turn into a commercial enterprise. if you do start charging or receiving any other gain then there would be lots of issues for the local council and also insurances.
You may want to consider insurance in any event. Even if you acted in the manner you describe you would be under a duty to exercise reasonable care and there could be a personal injury claim arising.