I hear what you're saying re cost BUT if you use a substandard product, all things being equal, you're likely to get a substandard result.
If your grandsons look at a "meh" finished product there reaction is likely to be the same - "meh". If however, you show them an eye popping finished product, chances are good that you'll peak their interest.
Bottom line - it's often best to buy the best products because they yield the best results.