The ants wont eat your Tomato plants but take a closer look Kaggy, there could be green/blackfly on your plants, the ants are attracted to the sweet honeydew these pests produce..

they can however builda nest in the grow bag and damage the roots while digging there hundreds of tunnels etc . if so just get some ant powder as that will sort them out or if its just because of aphids as youngdaisy sais id just use soapy water on them .