SQL - Return Last Update Date

I'm trying to return the LastUpdateDate based on a ParentID. This date can come from any of it's child tables. Let's say there are two child tables. Child one has a one to one relationship with the Parent and child two has a many to many relationship. See test tables below. Any help would be appreciated.

Parent Table

ParentID        Name      LastUpdateDate
1            Parent John     2014-06-26
2            Parent Mark     2004-07-27
3            Parent Bob      2009-04-07
4            Parent Jo       2014-09-26

ChildOne Table (1-1 Relationship)

ChildOneID      ParentID        Name            LastUpdateDate
10                 1    FirstChild Tom            2011-03-12
20                 2    FirstChild David          2014-08-11
30                 3    FirstChild Sally          2009-04-06
40                 4    FirstChild Jane           2014-11-26

ChildTwo Table (many - many Relationship)

ChildTwoID      ParentID        Name            LastUpdateDate
100                 1   SecondChild Phil           2014-03-12
200                 2   SecondChild Smith          2012-08-10
300                 3   SecondChild Paul           1999-04-06
400                 4   SecondChild Ed             2010-11-26
500                 2   SecondChild Donna          2010-08-10
600                 4   SecondChild Melissa        2008-10-16