We used a philosophy similar to those others discussed. We had an upper level that we could afford, approximately equal to our in-state flagship COA. Fortunately, DS earned a merit award that made it less for us.
We used the phrase “You can apply anywhere, but we can’t necessarily afford everywhere. Get into a (financial) safety and then go for the longshot merit awards.”
Whatever the budget is, try to articulate it to your DS/DD in a way they can understand and accept, even if it’s not ideal.
Oh, and on the multiple child issue, “That which is fair is not always equal.”