There's nothing in Debrett's about marriages or children for either one of them.
The good news is that there are some young heirs, born in the 21st century, besides the aforementioned Frederick.
Andrew Parker Bowles' brother Simon has a grandson Oliver (b. 2011, son of Luke (b. 1978).
Also, from another branch of the family, there are 2 others. They are descendants of the 9th son of the 6th Earl:
1. Philip (Buffy) b. 1955
1.1. Archie (b. 1987)
1.1.1 George (b. 2020)
1.1.2. Frederick (b. 2021)