mysql - замена значений столбца их соответствующими значениями из 2 разных таблиц

Вам просто нужно снова присоединиться к таблицам credentials и email , чтобы извлечь родительские данные:

select
    cr1.data                as "Parent Name",
    e1.email                as "Parent Email",
    cr.data                 as "Child Name",
    e.email                 as "Child Email",
    sum(a.amount)           as "Amount",
    (sum(a.amount)*2.5/100) as "% of Amount"

from 
    email e
join
    class c
        on e.AcctID = c.AcctID
left join
    relationship r 
        on c.AcctID = r.child
left join
    amount a
        on e.AcctID = a.AcctID
left join
    credentials cr
        on e.AcctID = cr.AcctID and cr.value = 1
left join
    credentials cr1
        on r.Parent = cr1.AcctID and cr1.value = 1
left join
    email e1
        on e1.AcctId = r.Parent
where
    c.relationship = 'child'
group by 
    cr1.data, e1.email, cr.data, e.email
order by
    cr1.data

Вывод:

Parent Name     Parent Email    Child Name  Child Email     Amount  % of Amount
                so@so                       add@add         10.36   0.259000
John Smith      Test@Test       Jane Smith  email@email     24.72   0.618000