[{"name":"Peter","age":50,"job":"retired","children":[{"name":"Alex","age":33,"job":"teacher","children":null},{"name":"Jenny","age":31,"job":"teacher","children":null},{"name":"Rob","age":28,"job":"scientist","children":null},{"name":"Harry","age":27,"job":"teacher","children":null},{"name":"Tim","age":21,"job":"student","children":null},]}]

db.collection.aggregate([{$match:{name:"Peter"}},{$group:{_id:"$children.job",count:{$sum:1}}}])