Because of the child tax credit and the COVID recovery rebates, you don't owe any tax, so you can't use the credit. The credit will carry forward for 5 years, so presumably you will owe tax sometime in the future. (Although maybe not in 2021 since the child tax credit is increased to $3000 or $3600 for 2021.)
It's too late to increase your tax liability for 2020.
About the only thing I can think of for 2021, would be to start making after-tax investments or do a Roth IRA conversion.
1. If you are investing in a pre-tax account at work (401(k) or similar), if you switched to investing in an after-tax account (designated Roth account) you would lose the tax deduction and therefore owe more tax for 2021. But then when you go to withdraw the money in the Roth account, it will be completely tax free.
2. If you are investing in a regular IRA, switch to a Roth IRA for the same reason.
3. If you have an existing traditional IRA, you can convert some or all of the balance to a Roth IRA, which will be taxable when it happens, then your future withdrawals will be tax-free. Converting a regular IRA to a Roth IRA is probably the best way to increase your tax liability now, because of the long-term benefits.
4. If you have a 401(k) or other retirement account from a job that you have left, you can roll that over into an IRA, and if you make it a Roth IRA, that's also a taxable conversion which will increase your tax short-term for a significant long-term benefit.
You can also sell investments that have appreciated over time and use the solar credit to offset the capital gains tax. Then, if you buy new investments, you will have a higher cost basis and pay less tax in the future. To beware the Wash Rule -- you must wait at least 30 days after selling an investment before re-buying the same or similar investment.
