I am in this exact situation. Actually, an even more extreme version--I'm contributing to our 401(k) just enough to max my employer's match, and we're still WAY into negative tax territory with our child tax credits.
That said, your marginal tax rate is NOT 0%, unless your taxable income is $0. If your taxable income is $0 (line 43 on your Form 1040), then by all means dump as much as you can into a Roth/Roth 401k, and skip the rest of this post.
If your taxable income is over $0, you still have a marginal tax rate--10% if you're under $18k or so, 15% above that. Your tax, on line 44, will still be positive. If your taxable income is $17k and you reduce it (via increasing 401k contributions) to $16k, your tax on line 44 will decrease by $100 and your refund will increase by $100. Depending on your location, you might also have a state income tax that will increase your overall marginal tax rate anywhere up to 20% (if you're unfortunate enough to live in CA).
What do I plan to do? When we retire, I plan to move to a low-tax state, like TN or CO. As a result, I anticipate that my total (federal + state) effective tax rate will be lower than the 10% federal + 5% state I'm paying now. For me, that means extra money goes into the pre-tax accounts.