Improve syntax for entity types remaining_parent_state & remaining_child_state

[RN-Say]

Is your feature request related to a problem? Please describe.
The syntax of these entity types is difficult to understand, too long to type and obfuscates their function. As I read through the forum it was easier to understand the yaml early on when 'remaining_parent_state' was just 'remaining'. The counterpart 'remaining_child_state' is poorly named since it is a sum and not a remainder. The combination of the two makes it hard to quickly understand the yaml code.

Describe the solution you'd like
remaining_child_state > sum_children
remaining_parent_state > remaining

Describe alternatives you've considered
sum_of_children
remainder_prior_section
section_remainder
remainder

**Additional context
I think these small syntax changes would make the card more approachable for new and experienced users alike.

[MindFreeze]

It's not the sum though. It's the remainder of the sum in both directions. And remainder_of_child_sum/remainder_of_parent_sum doesn't seem like a big improvement