A card to accompany the birthday gift is not customary in Spain.
Birthday Parties in Andalucia
Traditionally, in Spain the birthday was not celebrated; rather, it was the "santo", or saint´s day, that was the day one celebrated becoming a year older. These celebrations were minimal compared to what they have become in recent years.
A simple "merienda" or afternoon snack, a little chorizo with bread and perhaps a cake with family and the friends in the neighbourhood was usually ample. However, nowadays the parties are more complete, though there are differing styles. Some people go all out and hire a bouncy castle and clown and have games and cake and gifts; others do it more simply, some snacks for all and time for the children to run around and play after a little cake. Many people go to the increasingly popular places that do children´s birthday parties for a fee, with the advantage that the house doesn´t get messy!
Depending upon how you have been raised, a few things about being the guest or the host in Spain may be a little different to what you are accustomed to. First, the invitations are often extended only a day or two in advance (and they may be just verbal); responding about whether or not your child will come is always helpful and courteous to whoever is planning the party, but there are many who do not do so and may show up, so as the host you may want to be prepared with extra party favours and plates. Many parents leave their children, even preschool age, and pick them up at the end of the party, while many others stay for the duration of the party (you may want to plan the type of snacks and drinks you offer accordingly). Guests´ younger or older siblings often will attend the party as well even if no mention was made ahead of time, so again, be prepared, but don´t worry too much about the games not being age-appropriate for all because the children always seem to find something to do (just in case, have on hand some paper and coloured felt tips to occupy any age). Regarding gifts, the idea of waiting for all guests to arrive and politely watch the guest of honour open them is a nice idea, but do not be shocked if the birthday boy or girl gleefully rips the gift open as you arrive with it. A card to accompany the gift is not customary in Spain, nor is the thank-you note afterwards, but both are always appreciated at least by the parents if not by the children.
Finally, timing is an important consideration. Children´s birthday parties are usually given in the afternoon, the exact time depending upon the time of year and whether or not it is a school night, in which case a snack and some cake are appropriate fare. Keep in mind the time that lunch is usually had in this country when planning your party times ... three o´clock in the afternoon would be right in the middle of lunch, whereas five o´clock is a much more common time to begin the party. Or, you can invite your guests for a full lunch, in which case a more appropriate time would be one o´clock, for example. Most parties last about three to four hours; but if you tell people to pick up their children at a certain time, do not expect special promptness, as things tend to dawdle on while the children have fun, which is really what it is all about.