This is really helpful, thanks yall.
Really small means ~$1,500-2,000/year. We don't even include it in our budget, so we don't count on it at all. Looks like that probably wouldn't qualify her for a SIMPLE IRA, but would work with a solo 401(k). Looking at the Vanguard solo, I don't see any minimums for opening an account, and the only account fee I see is $20/year per fund, which would be just fine with only one fund.
Am I missing anything? Looks like a fairly easy way to stash her earnings without much startup hassle other than a phone call.
This wouldn't add up to a ton over the years, but it would keep our tax bill down by $200-300/year as long as we funneled all of it to the solo account.