I am not sure I understand exactly what you are talking about.
If you are talking about good clothes that are no longer worn, shoes, toys etc and you sell them on Ebay, Craigslist, or just donate them to charity..... then yes, I would agree that you get the benefit of that transaction.
If you are talking about something more valuable...say a coin collection, car, jewelry, etc... then NO..... you should put those funds into an account strictly for your child.
In the meantime, don't rely on things you have given your children be to sold so that you can make ends meet.
If you are not too good with money, then make it your priority to get good with money. Go to the library, take a course, do something productive so that you learn how to be good with money. There are many organizations out there now that can guide you in finances, budgeting, etc that will help you tremendously in the future. You need to learn about money so you are able to properly provide for the children you have and your retirement.
Keep separate accounts for your children and do NOT touch that money for any reason.
Good luck.