The Under-23s regulars have penned two-year deals.
Roberts made 18 league and cup appearances for the Under-23s this season.
A left-footed centre-half, Roberts is swift and composed on the ball.
Thompson-Sommers, also aged 19, is a versatile midfielder.
He scored four goals in 23 competitive games for the Under-23s.
A left-sider who can link play well and likes to get forward, Thompson-Sommers joined Blues from Spurs in 2016.