They tend to have a lot more luggage room in terms of depth whereas an SUV has the advantage on the floor-to-ceiling height.
If it's picnic stuff, blankets, kids' beach gear and a couple of sun chairs, then you may find the station wagon is actually the better option. Downsides are the ease of entry and exit, higher ground clearance plus elevated driving position that SUV owners love. Look at a wagon with a tow bar because day trips may turn into camping holidays and you'd need a trailer.
Looking forward and the likelihood the loads will only get heavier, you may want to keep your engine search to no less than 2 litres.
Fuel consumption may suffer a little around town but, on the motorway with a full load, you will enjoy the drive more and fuel consumption should not suffer as much as a smaller engine pulling the same load.
Your budget also puts you in line to purchase a reasonably safe vehicle.
Safety started to become a major focus for manufacturers in the early to mid-2000s with multiple airbags and Electronic Stability Control becoming a standard feature as manufacturers chased a high safety rating in independent testing. It would pay to always check for a lap and diagonal seat belt in the rear seating positions however.
Lap belts are better than no seat belt at all but can cause severe internal injuries in the event of a major accident.
Try and also keep the odometer reading as low as can be. Ten years and around 200,000k is considered the benchmark for a car's life before costs can start to get away from owners.
A late model high mileage may turn out to be a higher risk than a vehicle a little older but with a lower odometer reading.
Mazda6 Wagon 2009
Honda Odyssey 2005
Ford Mondeo 2010
Engine sizes can vary from 2-l to 2.3-l but safety levels should remain extremely high including multiple airbags and ESC. A diesel engine is also available but, in your case, the recommendation is to stick with petrol.
Take the kids with you and let them experience the space and practicality that is on offer. Then do your checks especially if odometer readings are hovering around the 100,000km mark or over.