it depends on HOW you overestimated
if you overestimated your combined income, but underestimated the lower income earners income, you can still be overpaid ftbb - i have noticed that the system is using ftba supplements to pay ftbb debts BEFORE it uses ftbb supplement, so it can be confusing
if you have a low maintenance assessment at the start of the year and that goes up later in the year you can also be overpaid ftba and rent assistance.
if you happen to cop both of those happening, it's very easy to happen

there should be no reason why you got supplements for only one of your older children unless the other was used to pay back the overpayment you mentioned. they are balanced at the same time. your ex doing his tax (unless he was a current partner in the 09/10 financial year) won't impact payment of your supplements