I consider "How many grandchildren has she?" to be a very good way of asking that.
If "How much..." can be found in "ʻEhia(s)" drop down list, why canʻt it be used and accepted?
Because "how much" can only be used with uncountable nouns (like "how much water do you want"). Use "how many" with countable nouns, like grandchildren.
The drop-down hints donʻt always take context into account.