Standardise children field names in execution and planner explain outputs

in EXPLAIN and timing outputs, all the children fields should be called
"~child"
"~children"
"~first"
"~second"
so that they always come last in the respective subdocuments.
While this is generally the case - there's the odd circumstance where this does not hold true, and even worse, timings and EXPLAIN outputs are mismatching (one has the ~, the other doesn't).

Before fixing this we ought to make sure that the tests in test runner/pytests/tuqquery are not affected.

I have done a first round and I believe all the tests expect the ~, but a bit more due diligence is needed.