Self Join MYSQL Parent Child - mysql

Question

For some reason this is eluding me tonight. I have 3 tables, appointments, clients, tasks. I am trying to get all the appointments for today or tomorrow, check the users table to pull the current clients, and any family members, and then pull tasks that meet another criteria to get a full list of tasks for the day or past due.
On the client table we are importing information from another datasource maintaining the original id as reference_id, if they don't have a parent_id, they are the parent, otherwise they are children.
appointment table
id | user_id | client_id | start_at
1 1 2 2018-02-15
2 1 1 2018-02-15
3 1 2 2018-02-15
4 2 4 2018-02-15
clients table:
id | reference_id | parent_id | user_id
1 35 null 1
2 36 35 1
3 37 35 1
4 35 null 2
5 36 35 2
tasks table
id | client_id | task | due_date
1 2 do something 2018-02-15
2 4 do something 2018-01-10
3 2 do something 2018-02-01
4 2 do something 2018-05-15
I got started trying to join the table onto itself, but when I try combining the parent it ends up killing all results, but only when I add WHERE parent_id > 0 to the query. Before that I am getting about 40 results, but should be well over 100. In the dataset, I have 50 appointments, some of the users have 2-5 children / relationship but I am not sure why since I am doing LEFT JOIN instead of inner join, if anything I should start ending up with redundant extra results.
SELECT
start_at,
clients.id,
clients.reference_id,
clients.parent_id,
clients.first_name,
clients.last_name,
children.user_id,
children.first_name,
children.last_name,
children.reference_id,
children.parent_id,
parents.last_name
FROM
`reminderdental2`.`appointments` AS appointment
INNER JOIN clients AS clients
ON appointment.patient_id = clients.id
INNER JOIN clients AS children
ON clients.parent_id = children.parent_id
LEFT JOIN
(SELECT
reference_id,
user_id,
last_name,
first_name,
parent_id
FROM
clients WHERE parent_id > 0) AS parents
ON clients.parent_id = parents.reference_id
WHERE clients.user_id = '27'
AND children.user_id = '27'
AND parents.user_id = '27'
AND start_at > NOW() - INTERVAL 1 DAY
AND start_at < NOW() + INTERVAL 1 DAY
Is there a better way to design this than joining the client table 3 times? Why did it kill the query adding the WHERE parent_id > 0?