You are currently keeping all the money under the parent SSN/tax return. That means all the interest and capital gains are taxed at my marginal tax rates. If you kept it as actually belonging to the kids (joint accounts for the checking, custodial accounts for the brokerage) then it wouldn't be taxed at such high rates (in fact, most likely wouldn't be taxed at all). This costs a lot more money than your actual fees.

My suggestion is to keep the money & brokerage accounts in the actual kids' names rather than pretending to keep it there but actually having the ownership under the parent.