I was about to tell you to enter the distribution code 1 and Lacerte would calculate it but I see that is not working. Looks like you can enter the 10% tax in Screen 42 Other Taxes as "From Form 5329" and manually prepare and attach Form 5329. I'm guessing this will be a paper return since 5329 is not listed in efile attachments list..
Oops. Just realized you said simple trust. So all income is distributed and the beneficiary(ies) have to pay tax plus penalty. I guess you need to disclose in a statement on their K-1 that the amount on Line 5 (or the applicable amount) is subject to the 10% penalty on premature withdrawals.
My first answer would work for a complex trust or estate with no distributions.