Hi Henry,
What is the deposit meant? How much can I used? 100HK,50Hk or 150HK?

The HK$50 refundable deposit includes a card cost of HK$30 and a provision for negative value of HK$20 (of which the user can use it up to HK$35 once). Generally, you can use 100 HK dollars. When 100 HK dollars runs out, you still can use it up to HK 35 dollars without adding value.

So if I refund it whitin 3 month and my deposit still 75 hk i can get back 68HK is that right?

Yes, totally right.

Where can I buy octopus card and refund it? Is my child 5 year old must buy the ticket also?

You can buy an on-loan Octopus Card at the Hong Kong Airport Express station or any other metro stations. Also you can refund it at Customer Service Centres in MTR stations. You can buy her an on-loan octopus card for child use. The child octopus card is only available for 3 to 11 years old children. With this card, your child is admitted at half price when taking MTR and other public transport vehicles. By the way, it also includes 50 HK dollars deposit and the initial stored value is 20 HK dollars.