There's a thread on a possible adopted
child, and a couple
about the daughter in early Indy 4 scripts. If as hoped Ford and Rhys-Davies have large roles in the film, they will surely have younger (tween?) sidekicks to keep the cast balanced.
Maybe they've retconned Marion's early affair, but I still place her at 25 in 1936 and 46 in 1957. Brigitte Nielsen was in the news
for conceiving and birthing at 53, possibly with IVF (Louise Brown was the first IVF baby in 1978). Freezing unfertilized eggs is a recent advancement and still tricky.
Marion would have had a slim chance of conceiving. A daughter named after Indy's sister
perhaps? Total US births
to 45–49 year olds (unassisted by grail powered gonads):
1955 - 5,430
1960 - 5,182