Traditionally children take their mother's name. If the mother is married then traditionally said name would also be the father's name. If the mother is unmarried, then it would be the mother's name of birth.
Yvette Cooper and Ed Balls' children are Coopers rather than Ballses.
If you do go with Banga then avoid any name which can shorten to Ed, Ned or Ted - so Edward/Edmund/Edwin/Theodore/Nathan and Nathaniel.
Avoid names beginning with A, I and U.
I would avoid William as Billy Banga is quite spectacular.
I would go with something quite classic or currently popular:
Emily Banga
Sophie Banga
Lucy Banga
A classic longer name like Elizabeth or Katharine could also work.
For boys:
James Banga
Thomas Banga
Henry Banga
A common, or classic name will balance the unusual surname a bit.