precio de suma de niños en otra tabla mysql

Prueba esto;)

select ancestor_id as userid, sum(b.price) as total
from webineh_prefix_nodes_paths_tmp a 
join webineh_prefix_nodes_tmp b 
on b.userid = a.descendant_id
where a.ancestor_id in (select userid from webineh_prefix_nodes_tmp where parent = 1)
group by ancestor_id

Demostración de SQLFiddle

Editado

select ancestor_id as userid, sum(b.price) as total
from webineh_prefix_nodes_paths_tmp a 
join webineh_prefix_nodes_tmp b 
on b.userid = a.descendant_id
inner join webineh_prefix_nodes_tmp c
on a.ancestor_id = c.userid
and c.parent = 1
group by ancestor_id

Demostración de SQLFiddle