My family and I are going to Holland and then Prague in August. Does anyone know if we can buy SIM cards in Holland and use the same ones in Prague
The general-approach to solving this problem is identifying each country's carrier and see which carriers they have in common. In this Case T-Mobile and Vodaphone. Then check the carriers website for inter-country roaming agreement. Although it is not a guarantee that TMO or Vodaphone will have inter-country agreements between the same carrier, it is also possible to have inter-company agreements.
That being said, I have used an American TMO account in the NL, Italy, FR with a roaming agreement where it does not make economic \ functional sense to buy a SIM card. This is because texting \ 3G data is included at no charge and Whatsapp (data) is used with relatives. Telephone roaming calls are at .20 USD per minute. Wifi telephone calls are free too. This American TMO plan also includes the Czech Republic
gatorback has the correct answer here. Another option is to get hold of an international SIM card, which will work in numerous countries. You can get these online (sellers such as Telestial, GO-SIM, WorldSIM and others) and they'll deliver the SIM to you before you travel. Check each site carefully for the rate information, as one may be better for you than some others.