You wouldn't want to use a "Category" in this case, you'd use an "Account" or "Accounts." (For clarification: In Quicken-speak a "Category" is what real accountants would refer to as an "income" or "expense" account. That is, you'd expect to see "Categories" listed out in a Profit or Loss statement or an Income and Expenses statement. Quicken "Spending" reports typically list out "Categories." An "Account", on the other hand, is what real accountants would call a "balance sheet" account; some sort of asset, or liability. Quicken "Net Worth" and "Account Balances" reports typically list out "Accounts.")So, in the case of son "Joe" who says "Dad, buy me this and I'll pay you back" you'd set up some sort of asset Account - a "receivable" - titled something like "Due from Joe" and when you bought whatever it is he wants you'd record this as a "transfer" - another bit of "Quicken-speak" - out of your bank Account and into the [Due from Joe] asset Account. That would establish the amount that Joe owes you and anytime you wanted to see what that amount was you'd simply run a Net Worth or Account Balance report and that amount would be plain to see.If Joe actually paid back some amount he owed you you'd enter that in Quicken as a deposit to your bank account with the offset being the [Due from Joe] asset Account. That would update the amount the Joe owes you and that amount would be readily visible anytime you ran a Net Worth or Account Balances" report.You'd set up an asset Account for each child that "borrows" from Bank of Dad and make your accounting entries into these Accounts for amounts you've spent and amount that each child pays back. In your will you'd specify that the Account balance for each child gets settled up before the property division.
I'd just go with a category. Set up a category called 'kids', with each one in a sub category, & when you go through each purchase, assign it to whatever kid has made the charge. You can make an easy quick report, or have more detailed reports that add up over the years. If one of the kids has a weak moment, & pays you back some of it, you can have that as a positive amount in that category.