get_children() is not cached on the VIP GO Platform but also performs a no-LIMIT query by default.
Any no-LIMIT query will have scaling issue as the database increase.
How to solve this issue:
Setting posts_per_page to 1 is already a good start as it will take care of the no-LIMIT issue
Going further using a WP_Query like the one below to fetch the last image attached could be more optimised.