expose a function that queries only the children of a node, not the n…

[brookeangel]

Motivation: https://github.com/eeue56/elm-html-test/compare/master...brookeangel:children-query-fix?expand=1#diff-f24cdf4fdbc01dd4d5db0a9c1723a9e6R287

• In an initial merged PR, we exposed getChildren
• What we actually needed to expose was queryChildrenOnly
• See this branch - this fixes the bug but with using code that ought to live in elm-html-query
• If queryChildrenOnly is exposed, we can fix the bug with much less code in elm-html-test
• Since getChildren was never published, this can be merged and published as 1.2.0
• Sorry for the mixup last time, hopefully this is more clear!

Paired with @rtfeldman