how to get count the events of each topic's child from another table?

Question

i have 2 table's.

table TOPICS
topic_id | auto increment
topic_pid | int => points to his parent
topic_name | string

table events
topic_id | int => points to topic table
title | string

i want to get results that show me each How many events each topic and its children have?

for example

TOPIC‌ TABLE 

————————————-

topic_id | topic_pid | topic_name

  1           0         product

  2           1         downloadable

  3           2         movie

 EVENT TABLE

 ————————————-

 topic_id | title 

   3         buy

   3         like

   2         like

 

my ideal answer:

  name      | event count
  product        3
  downloadable   3
  movie          2

because movie is downloadable's child and downloadable is product's child

my topic child have child