Try to get as much childcare experience as possible before departure. Doing some volunteer work at a childcare center is very helpful and families love it.
Get material, books, ideas about games, stories, crafts, activities, songs, etc… that you can use while you are with kids. They help a lot to improve their language skills (English or other…) and the most important thing, they do not get bored with you! You can bring them some story books as a present.
Try to bring some ideas about baking for kids, they enjoy very much these kind of activities.
Get some skills in baking. Children love doing cookies or cakes or cupcakes with their au pair, and it is a good way to keep them busy and happy.
If you are not very familiar with housework, ask your mum or someone more experienced about main tasks to be done in a busy house with kids and try to get some practice with them. Spanish families normally do not expect you do a lot of housework as their priority is that you teach the language to their kids. They normally have a cleaning lady doing most of it, but they want their au pair to be responsible, clean, tidy. They do not want you to be more workload for their cleaning lady. Someone that also teaches the kids good habits.