Do you have a carte "famille nombreuse"? If you pay a few euro for one, adults get 50% prices at all times, not just at weekends on return fares. Children under 12 travel free at all times, whether with you are with someone else. 12-18s also 50% at all times.
It might be cheapest to split the ticket up to get the cheapest fare. I am assuming you live in Brussels. Brussels-Antwerpen costs €7.60 return per adult/over 12 if you have a carte famille nombreuse. Antwerpen-Amsterdam separate ticket but if you choose the right train, you stay on the train. Also check the price for a through ticket, the saving for a split ticket might not be so good. Children under 12 cost just €5 return on IC trains. A non promo adult ticket might cost perhaps €70 return starting price?
It probably would be a bit cheaper to go by car if calculating the cost of fuel only, if you cost journeys by complete running cost of a car, you could be looking at a more expensive trip by car, depends on how you look at your car costs.
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