Organizing large child tables & indexes for quick seeking

I have a parent table which at the moment has 140 child tables, each
partitioned by year and week (named table_yr2018_wk25, etc). Each child
table has something like 80-100m rows.

Querying from the parent table can be a hit or miss operation: sometimes
the planner does a sequence scan over each child table and its millions of
rows (which of course never completes) and sometimes it uses an index: I
have an index on each child table which is comprised of 5 columns,
including week and year.

The PK on the parent table is a uuid.

Any tips here for ensuring a faster seek to the appropriate child table?
Should I make an index that just has week/year?

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