Hahaha, I’m about one/one and a half head taller than your youngest. I don’t remember any player equally taller than me ever complaining about playing against me or any other equally small player for those reasons. (And yes, I get that the difference between 1,80 and 1,40 is a lot bigger than 1,80 and 1,60. But still, it should be manageable on court.)
So why complain when the person it affects is young? This is a very adultistic view, not at all inclusive.
I hope that the community in general welcomes players of all ages. LeftyPaul has a point though. Organisers can make their own decisions. But I hope their argumentation isn´t common ground for the community because it makes me very concerned for everyone’s safety (“The argumentation went somehow that players in (certain?) tournaments want to play more competitive and do not want to be concerned about taking care too much of other players safety.”)
If that argument was true (which I hope it’s not), everyone else seems to be a bigger threat to safety on court. I saw your kids play and there is no reason to believe the games are unsafe when they’re on court. They certainly know the rules of the game and have great bike handling, including your youngest. These things alone contribute a lot to safety on court. I’ve seen adult newbies make games a lot more unsafe on the court!
In the end we´re all responsible for the safety of the players around us. Being it by not drinking or consuming substances, by playing by the rules, by leaving people a bit of space if we see their bike-handling is not yet good enough.
P.S.: Speaking as an organiser, I can only emphasise to other organisers the importance of checking with the parents/legal guardians of a minor player. Minors should either be accompanied by their parents/legal guardians, who have to sign the waiver for them. If parents/legal guardians can´t tag along, they need to nominate a person in charge of their minor in writing and also leave their contact details with them.