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Them being God's children by the promise is not the issue.
He does not offer to those things which are not his children, or called out ones
The issue is God gave them of His grace free manna to eat. That free gift came with the conditions the Hebrews do the work of gathering. So did their work in gathering mean they earned the manna? No. That work was simply a condition attached to the free gift of manna.
No. That is not the issue.
the issue is God did what he did to us all. He offers us blessings, by grace (we can never earn it) and asked us to take it. It is NOT A WORK TO TAKE IT.
your trying to mix a condition of someone recieving something, and equating it with works. Does not compute..
next.