The EU, NATO, Japan, etc didn't start using that designation for nothing.
PKK also had an estimated 3000 child soldiers which were also used for terror operations and in combat in Syria: https://hrwf.eu/wp-content/uploads/2016/07/Child-soldiers-in...
The radical Kurds aren't going to win anything by engaging in it nor will the more moderate Kurdish groups gain anything by quietly supporting them.
Turkey has much to lose in the years to come by exposing their soldiers in Kurdish majority areas and they are only further alienating themselves from Europe and the world, especially if they get too zealous in the process. They haven't even faced any real resistance yet since they focused on Arab areas first. Their decision making won't be without cost.
There's no way this will be easy ride for them. Nothing in the middle east is ever like that. This is just downtime between the next major conflict. There's only so much to gain for anyone involved.
It is not logical to call some people terrorists when they are being massacred by a large professional army and try to defend themselves by whatever means they have. If the kurdish nation exists in 50 years, today's "terrorists" will be remembered fondly, just as the glorious french resistance is.
The border security argument is a bit weak. You shouldn't need to cross a border to defend a border.