Reduce their pocket money by half. Explain that there is a financial crises in the economy in real life people can lose jobs and be forced to take pay cuts)
The idea is to make sure they cannot repay you back with the new pressures you are adding on. You can also increase interest rate to 70 % or more and demand the same repayment tenure, so they have to pay you back more, if you can explain interest rates in a way they can understand for younger children, take one and all them if thney borrow this they have to pay back interest
During this period make sure their health is not compromise. Ensure that they take food from home for school breaks. You can even communicate your pant and what you are doing to your child teachers so they can keep an eye on your child and work with you.
You can also taje money from next weeks pocket money if they are short so tell them they get nothing the following week.
They have to eat at school, so you tell them that ou have to responses the purchased item temporarly. If they cannot pay you back. Put it somewhere where they can see it, but do not let them paly with it or use it. However, still do not give the reduced sum that gets less and less.
The idea is to change the rules and pile on pressure to repay more that what was initially agreed. If they complain, bring out the agreement. Keep raising the interest rates little until they cannot repay and have to completely forfeit the item to you.