It took a decent amount of searching on the internet before I found something that would work, so hopefully this can save other people some time.
Making passport stamps for kidsI used a photo of blank passport stamps from istock and resized it in Microsoft Word to half the size of an A4 paper. Then I inserted various text boxes and word art to write the country's name in 10 of the stamps, while leaving room for a date stamp. Since I only needed ten stamps, it didn't matter that the middle of the photo was covered in an "istock" watermark; I simply didn't use those stamps.
Also, since mine would only be printed in black and white, I didn't change the font color. If you print these in color, you could make them look a lot better by playing with font colors.
Here's a word document of the editable passport stamps.
And here's a picture of the stamps in a student's passport, with the date stamp:
International passport templateAnd here's where you can download the blank passport templates I used (black and white) for the kids.