For some data formatters, even getting the number of children can be an
expensive operations (e.g., needing to walk a linked list to determine
the number of elements). This is then wasted work when we know we will
be printing only small number of them.
This patch replaces the calls to GetNumChildren (at least those on the
"frame var" path) with the calls to the capped version, passing the
value of `max-children-count` setting (plus one)
763b96c86d