I have a young man (age 7) who is in my Boyscout Den. I called his home recently to remind him of when our meetings would start this fall. His father a US Citizen let me know that his mother (England Citizenship) took him and his sister (age 4) to England without his consent and says that she might or might not bring them back in January. The father would like his children back, who should he contact and how does this process get started?

He needs to hire an attorney just like in any divorce or child cusody cases.
The United Kingdom is easier, since we have good political ties with them, but of course if she has joint custody she did not need his consent to take her children also, that is up to exactly if they have any child custody agreement and if so what it says.

But it will involve getting a court order here, then applying to a count in UK to honor that court order or issue one of their own also.