Kayo, a 17-year-old midfielder from Maryland, has represented the U.S. at under-15 and under-17 levels. He was a member of the country's U17 World Cup team last fall, playing in one match at the tournament. He joins Wolfsburg from Orange County SC.
Full-back Hernandez-Foster, 18, was also a member of the under-17 World Cup squad and has played 22 times overall for the team. He moves to Wolfsburg from LA Galaxy II.
The pair will join three other Americans at the club. Ulysses Llanez -- also a former LA Galaxy Academy player -- and Michael Edwards, have both established themselves in the reserves team. And longtime U.S. international John Anthony Brooks has been with the club since 2017.
Wolfsburg finished seventh in the Bundesliga this season, securing a Europa League group stage place.