Querying w/ join slow for large/many child tables

I have a primary parent table with a child table per week of the year for
each week back through 2015. There are a lot of child tables. Each week's
child table has maybe 80-110m rows.

When I join to the parent table on a column, it's very slow, but when I
manually specify the specific week's child table, it's quite fast, e.g.

Slow:
select * from foo
join schema.mytable on foo.col = mytable.col

vs. fast:
select * from foo
join schema.mytable_2015_wk33 as mytable on foo.col = mytable.col

What's the resolution to make querying the table faster in terms of it
finding the appropriate child table? Is it putting an index on `col` on
each child table? Some other thing?

Thank you.

--
Wells Oliver
wells(dot)oliver(at)gmail(dot)com <wellsoliver(at)gmail(dot)com>