This would be pronounced "DESS-im-uh". Personally it's too close to the word decimate for me. When Romans decimated the enemy, they lined them up and counted each one up to 10, and then whoever was 10 had their head chopped off and the count started again.
I think this would only be fitting for a tenth child, or maybe if she were born on the tenth day/month/etc. It's just too prosaic and literal-minded in my opinion (or I'm not imaginative enough to see a creative side to this number-name).