Southgate, who ultimately saw England beaten 2-1 by Croatia in their first World Cup semi-final since 1990, said he was fortunate to have a great back-up team - his assistant Steve Holland was also awarded a new contract, and the players performing on the pitch.
"Very often we hold youngsters back a bit too much", said Southgate, who previously coached England's under-21 team. "I'm always trying to find that balance, the messaging has to be correct, but it's good for their development to see how they train with us".
Southgate has made a strong impact since taking over from the sacked Sam Allardyce in 2016, evidenced by England's performances at Russian Federation 2018, when the Three Lions reached its first World Cup semi-final since 1990.
"Well, it is (a big call) but I think it aligns with what we've done over the last 18 months, really", the Three Lions boss said. We have a few injuries in midfield in particular so there was an opportunity there.
"[Sancho and Mount] are both in their formative stages but we think they can play in the group".
Teenager Prithvi Shaw roars at Rajkot with Test century on debut
Brief scores: India 364/4 ( Prithvi Shaw 134, Cheteshwar Pujara 86, Shannon Gabriel 1/66) against West Indies on Day 1. He was not facing one of the best attacks but he gave ample evidence of his brilliance on way to a memorable hundred.
Boothroyd told Football Paradise: "It's probably a little early for the boys but what we have seen is a strategy evolving that if players do get injured or come out of the first team, Gareth immediately looks at the under-21s". He played previous year on loan at Vitesse [Arnhem] and has had another season of men's football and created more chances than any other player in the league. He recently signed a new contract with the German club. The Dortmund front-man's last appearance for La Roja came over two years ago, but after scoring four goals in just 140 minutes this season, he has been included in Luis Enrique's squad.
The 21-year-old has scored three Premier League goals and assisted a further two since his summer move from Norwich City. The selection of Sancho and Mount, as well as set-piece specialist Maddison, is Southgate's way of confronting that problem.
Fulham keeper Marcus Bettinelli, Southampton counterpart Alex McCarthy and Watford midfielder Nathaniel Chalobah are the other uncapped players.
England kicked-off their UEFA Nation League campaign to a bad start as Southgate's men lost at home to Spain.
Tottenham midfielder Harry Winks, whose only cap came against Lithuania in October 2017, returns to the squad.
Southgate, who became England coach in November 2016, is contracted to stay in charge of the national team through to the 2022 World Cup in Qatar.