Has anybody transited through Frankfurt, on the way to UK, with their Thai family; will they need a transit visa?

Connecting flight only. Thanks

When they arrive from a non-Schengen country they will have to go through passport control like any other passenger arriving from flights that originate in a non-Schengen country. After that they can immediately go to their intra-Schengen boarding gate with no additional passport checks. it is basically like hopping on a domestic flight.