Hi Ashleigh
I think I have the same unwelcome guests dining on my brassicas. On the basis of their habit of jumping off disturbed leaves I searched “flea beetles” on the RHS website and in my case I think thats what they are.
You could try Google images for pictures of flea beetles but be warned that lots of different species on lots of different crops appear to have picked up this name.
I covered the neighbouring bed of autumn salads with enviromesh after planting and then also a layer of fleece as my mesh apparently may only deter the little darlings. So far so good!
CP