how can i book a taxi for 2 adults and 3 children from capital airport to downtown ?

You can call 96103 for booking the taxi. The taxies are waiting in No.1 Gate in the first floor of T1, No. 5-9 Gate of T2.

However, it's not allowed for 5 people to share a car, so you are suggested to take two. Or you can book the transfer service since you have three kids. Then TravelChinaGuide is recommended and you may contact them by bookingtravelchinaguide.com.