Am I correct that a parent table must precede a child table (in a DDL statement) or get a errno150?

That's my conclusion after several rounds of isolate and substitute.

BTW -- I had no problems (no errno 150) with a child table preceding a parent when building MyISAM tables. Is that because ref. integrity isn't forced with the MyISAM engine, or because something else was different?

you understand the situation perfectly

a foreign key cannot reference a primary/unique key if that key has not yet been defined

and to answer your other question, since myisam tables don't enforce referential integrity you won't run into the same problem with them that you do with innodb ones.