One of the practical considerations when traveling with small children is, what the heck do we do about car safety seats? What are the laws in Europe? Granted, most of the time we'll be traveling by train or bus, but we are planning to rent a car at the Malaga airport, so for that we'll definitely need them. The last time we trusted a car rental company to provide suitable car seats (Alamo rental at the Tucson, AZ airport) we learned an important lesson: take care of this detail yourself if you want any control over the quality of the seat!
Anyway, back to the law. I asked my friend M from The Netherlands what the law is. She said in The Netherlands, both of my girls would need to be in car seats, at least a booster for JieJie (almost 5) and a full-fledged restraint for MeiMei, who is still wearing size 2T at age 3 1/2. But, she said, who knows about other countries as there is no EU-wide law as of yet.
I did a little scouting around and found http://www.childcarseats.org.uk which has a handy compilation of the car seat laws all over Europe. Basically we need to have car seats for the girls unless we are in a taxi.
Usually I like to travel as light as possible, but we're really going to need their seats with us...to get to the airport, to get from the Brussels airport to Antwerp by van, and in the Costa del Sol definitely. Looks like we're checking two car seats. M offered to find some second-hand, but we'll need them before we see her, so best to bring ours.