I would suggest that ex-JW activists avoid doing the same thing even if it is something that the children enjoy
I generally agree with you, but its not that we want to engage kids in the protests to grab attention. If they are there it is usually because the parents wouldn't have anyone to leave them with while they're away, especially because some of those people traveled quite far to participate. Parents of this boy with a sign came all the way from Berlin, Germany (about 500 kilometers away from where the protest was held) and they didn't really have any family to leave him with back there.
I am sure that it has helped untold numbers of Polish JWs to give careful thought to what is happening in JW organization
This is already happening in Poland I think, the latest Yearbook showed a -2% decrease, second largest world-wide. Factoring in the baptisms, more than 3500 JWs left the org during last service year. Even if you say that 1% died, that's still ~2500 who left. And this trend is not new, Poland has not seen any growth for last 10 years :).