How do I use grep to find a text string in files in sub folders of a parent folder without -r switch

The standard (POSIX) syntax is:

find /path/to/parent -type f -exec grep 'XXX' /dev/null {} +

(the /dev/null is to make sure grep always prints a file name). That will work on all POSIX systems including Solaris. The only known post-90s systems where that's known not to work is old (very old now) GNU systems.

GNU initially introduced a -print0 predicate and a -0 option to xargs instead for that:

find /path/to/parent -type f -print0 | xargs -0 grep 'XXX' /dev/null

There are now a few other implementations that support that, but not Solaris.

_{Above, in theory, you'd want to add -r option to avoid running grep if there's not file, but that's not as portable and in this particular case, doesn't make a functional difference}