Make sure you have a GHIC for every member of the family. Even if your travel insurance includes medical cover, the Spanish public system (which is very like the NHS) is there to do the heavy lifting if needed. Before you go, look up where the nearest public health centre (this may be called a PAC, a CS, or a UBS) and pharmacy are to your hotel. In some places the hotel receptionists are on commission from local private clinics to refer sick tourists to them, or to call a private ambulance (€€€) rather than a public one. 😠The emergency number is 112.
The pharmacies are good, but over-the-counter medicines are different to what you are used to, so take your own if you have strong preferences.
Bring back a tube of "Cuatrocrem", which is a skin cream with four active ingredients (anti-bacterial, antibiotic, antifungal, cortisone) that fixes absolutely any non-specific problem that you can't be arsed to go to the doctor for.
If DC is still in nappies, Pampers are called Dodot in Spain. 😉
You should expect to encounter a few mosquitoes. Your hotel might provide night-time plug-ins, and if not you can buy one for €6 or so in a Spar convenience store, or €3 at Lidl. You can also get after-bite sticks in any pharmacy.