agree, also make sure the letter includes yours, the mothers and childs, full names, dates of birth and passport numbers, so that the letter can only be used by your family. and take the birth and marriage certificate as well. Unlikely to have a problem, but better to have more documentation than needed, and proof of your consent for specific persons.

P.S. Don't bother with the UK Embassy, worse than useless to the point, if emergency situations arose, I would try to get help from another commonwealth country first.