If you prefer to arrange your transfers before you go, there are various companies such as
ResortHoppa, and
Shuttledirect that you can book with these to give you that peace of mind that you will get to your hotel or apartment easilt and swiftly.

There is a bus service that meets most scheduled flights and charges a few euros per person for the transfer and will drop you off at your hotel, but remember this may take considerably longer than a taxi depending on where your hotel or apartment is. This service can also be organised to pick you up at your hotel to take you back to the airport, although I have heard quite a few stories of the coach turning up late or not at all.
There are also several bus services that take you to Tarragona and Reus, after the shortish trip, the bus drops you at the main Reus bus terminal where there are ongoing buses to Salou, please be warned there can be a lengthy wait for a bus at the bus terminal in Reus which may be troublesome with children. It also has to be said that the Reus bus terminal is not the nicest of looking places, although the last time I was there, there was work going on updating the bus terminal. There also seems to be a certain lack of taxi's anywhere near the bus station.