Mole is a bit hot for children and it may prove difficult to find. But something very mexican that few people acknowledge is chocolat... it was invented in Mexico (Xiocolatl)the same as chewing gum (chicle). So those are some surprising options.
Something that you can do is Enchiladas, they sell the sauce in most Tescos/Sansbury (it comes in mild and hot varieties, Old El Paso is IMO the better brand). You only need to get a white mild cheddar and shred it and 2-3 packages of tortillas.
Just roll some tortillas with the grated cheese inside (about an inch diameter) and put them in an oven tray. Heat the sauce in the microwave slightly then cover the rolled totillas with it, and sprinkle more shreded cheese on top, get it in to the oven just to allow the sauce to simmer and the cheese to melt.
You can cut the rolled tortillas in three before adding the sauce, which is more or less the size of a child's portion and makes the dish far easier to serve.
In terms of deserts.................. (thinking).......... I really don't know as we eat more or less what everybody will eat in the rest of the world.... there's arroz con leche but probably a Spanish person with more experience in the kitchen could provide a good recipe. {whymummy... how do you do this????). there's also capirotada which is a bread pudding that is typical of lent and Easter seasons. But again, I have never made it. But will try to get a recipe for you.
Regarding the pinata, this is how original pinatas looked like, they were used only for the 9 days before Christmas. they are very easy to do. Just get a big ballon, newspapers, white glue and coloured tissue paper (like the one used inside gift bags). You just need to cover the ballon with strips of news paper and let it dry, then roll some newspapers into cones and stick them to the side, then cover all that in colourful paper. Once thepinyata is finished and dry you cut a little hole to allows sweets inside the ballon, donth forget to add a good piece of rope, wire etc, that can suport the weight of the pinyata while hanging from a rope. Any stick could do, some mothers decorate a broomstick with the same colours of the pinyata but to be honest I have some faint memories of ocassionally using a baseball bat!