I am in exactly the same boat in the kids band I play in. Normally a 5 piece, they have a sliding pricing scale of duo (vocals and either guitar or keys), trio, quartet (add in the drums) or quintet with me. I reckon I do about 30% of their gigs. Our situation is of course a bit different - charging over £500 for a full band for a kid’s 8th birthday is probably a bit steep for most (even here in Surrey 😜) hence the sliding scale, I just find it quite irritating that the bass is the first to be dropped. If there’s a date I can’t do they don’t always bother getting a dep, they just take more money each!
So... I know how you feel. I’ve just decided to suck it up. Until I hit 40 and quit because it’ll be a bit weird