Well, you’ve raised a very knotty question here. You had a bad experience in Belgium, but the fact is that most campsites are pretty safe, and most campers are very good people who don’t steal or threaten.
That said, a tent is obviously very vulnerable. It’s easy to slash and take things out. It’s perfectly understandable that you want to feel safe with two young children.
Really, the best advice is to use your common sense. Find out what you can about the campsite before you book a place there. Read about it (you might well find reviews of it online), and if everything seems positive, then go ahead.
Don’t pitch your tent in a secluded spot – that’s offering temptation to thieves and others. It might be noisier, but it’s also safer to be around other people. In general, thieves won’t operate where they’re likely to be disturbed.
Some people put a padlock on their tent as a deterrent. Certainly it stops someone ducking into the tent, which is a good thing, but it won’t stop anyone using a knife. However, the theory is that they generally won’t do this, as it’s quite noticeable.
Keep any valuables out of sight, and don’t leave them in your tent when you’re not there. Campers might generally be honest, but there’s no sense in putting temptation in someone’s way. Even when you have your valuables in your car, keep them out of sight, either in the glove compartment or under the parcel shelf – and, of course, keep your car locked.
From your experience, you’re going to feel most vulnerable at night, but unless you stay awake and on guard, there’s very little you can do. For what it’s worth, the odds are very much in favour of you having no problems in the campsite. In that regard, there’s little to distinguish Europe from the UK; campsites across the continent tend to be very safe, and mostly crime-free, places.
Obviously, you’ll be nervous, but don’t let one bad experience rule your life. Talk to other people who’ve done plenty of camping, and you’ll find that in virtually every case they’ll have had great, problem-free times, and that should reassure you.
Be aware of your surroundings, be cautious, and you should find that you have a lovely, uneventful camping trip.