When this sad day comes, I like finding ways to make 9 Av meaningful with our kids. Usually our activities include (or newly include this year):
1) making an "Unconditional Love Will Rebuild the World" - "Ahavat chinam yiboneh et ha olam" poster- this year it will be especially working on the kids' "sisters" relationship. We're really trying to encourage the kids to treat each other like friends, not rivals. We frequently say, "that's not how friends treat each other" to help remind them how to play nicely. We also say, "your sister is the friend that Hashem gave you in your family! She can be your friend forever." (That was a digression, but for a worthwhile cause...)
2) This year I thought of doing a short skit of the Kamtza/Bar Kamtza story, and they will either dress up for the skit or use these popsicle stick puppets:
3) Here are a few videos that I found new this year for kids- there are probably more. There are certainly tons for adults.
not really for kids but has cute graphics: https://www.alephbeta.org/course/lecture/tisha-bav-2014-lecture-1
4) An idea from Mishpacha magazine: Make hands and hearts bracelets. Holding hands and hearts together, we’ll bring the Beis Hamikdash!
5) We videotaped the girls and their friend doing an "Ahavat Chinam, Sinat Chinam Theatre" where they put on my hats or scarves for basic costumes and made up their own scenes of kind acts or nasty acts. The audience (my sister, "Doda Marie" and I) had to guess what they were doing. They got to watch the video of themselves while they colored the puppets/poster.
Here's an article on Ahavat Chinam- "free love," if you will: