I chose this font for both the invitation and the valentine because it feels very celtic and it's super easy to read. I had found some other fonts that were a little more stylized but were not as easy to read. After all my target audience is 5 and 6 year olds. Plus, you can't go wrong with anything in the Garamond family.
You can download Celtic Garamond from dafont, fontzone, fontspace, or fonts4free. If there is a site you prefer to use over another than download it from there, otherwise here is a link to download it from dafont. *This is free for personal use only. If you plan to sell something using this font you'll have to pay.
Here is a reminder of one of the cute valentine cards I used it in.
and here is the invitation I made using the font.
What do you think?