The Demo account is a fictitious account. This means that the balance shown in the Demo account is not real money. Since the amount in the Demo account is not real money, it is not possible to get the amount paid out or to withdraw it as cash.
The demo account is used to keep track of how much money parents "owe" a child from un-paid allowances. If you are a child that uses Gimi, you can therefore talk to your parents about the best way to get the money your parents "owe" you. Either they pay you out in cash, or they pay for what you buy, you can then deduct the amount you received from your parents from the demo account.