I used to keep two different blocks of Japanese curry roux, one was a “sweet mouth” mild version, another was hotter kind, because my daughter couldn’t tolerate a moderate spice level. Rather than make two batches, I’d make it with “sweet mouth” roux first, scoop out a serving for her, then add the spicier roux for us adults. She’s caught up with us now (to a degree) so I no longer need to buy the mild style of curry roux.
Personally, I think that merely adding intense chilli oil to a mild curry isn’t enough, curry should be about balance.

First of all, have you tried Japanese Curry in different heat levels?
Here’s a post by a regular contributor to the Japan Forum who prepared an “extra hot” curry in the USA:
https://www.flyertalk.com/forum/32260373-post58.html
Even with an “Extra Hot” curry roux, he still prefers to customise it a little with extra flavours.

You also have kids at your table, so another trick to individually adjusting heat levels for a bland curry is the accompanying pickles or chutney. Japanese curries REQUIRE a sweet/sour accompaniment. The standard in Japan is called Fukujinzuke, but we’ve had Atchara recommended to us (@Gradfly) and I suggest pickled beetroot. But you could use really spicy condiments instead as a way to introduce heat - a spicy beetroot relish or an intensely spicy mango chutney would work.