Because she was actually tailing him from the start. Taber, being a double agent, was aware that Schroeder had kidnapped Donna and knew Duke would be coming for her. Remember Schroeder said he'd known Lloyd was in Dallas for some time. This he knew from using 'his' agent in the CIA: Taber. From Taber's point of view Schroeder's deteriorating health and Lloyd being tucked safely far away from Berlin in Dallas was enough to ensure his safety. Donna's trip to Europe and kidnapping put all that in jeopardy.
From there it would be no problem knowing Chris was coming with his father. The meeting in the hotel restaurant was all an act on Taber's part; he already knew they were there and the first attempt on Lloyd had failed.
Carla & Glasses were either CIA working together for Taber, or more likely freelancers for him, (might be tough explaining to higher ups why he was trying to kill a retired agent searching for his kidnapped wife - easier to freelance the hit and blame it logically on Schroeder getting his revenge).
All of Carla's interactions with Chris were designed to distract him in someway from aiding Lloyd while Glasses tried to kill him.
The first time was when they arrived in Paris, she bumped into Chris to get his attention and distract him from the intended assassination and/or kidnapping of his father (which was then foiled by Schroeder's man getting to Lloyd first).
Then enticing him him to her apartment in Berlin to keep him from meeting up with his father in Berlin. If he'd gone on to Frankfurt instead he'd be out of position to aid Lloyd in the endgame, so mission accomplished anyway. Win/win for Carla, only to be foiled by Chris's ability to turn her seduction down.
Finally directly exposing herself to keep him in place in the cafe courtyard at gunpoint. Killing Chris was probably a last resort (again to keep from having higher ups digging too deep into Taber's activities), but would have been required after Carla revealed herself after having to so obviously tailing him - so not really a case of her out to get him, at least not at first.
Carla and Glasses knew Lloyd's and Chris's every move from Taber after his meeting with Lloyd, who was unknowingly keeping him up to speed (in fact the only time Glasses was ahead of Taber on intelligence was when he learned about Lloyd's plans from the Colonel first). That's how Carla kept 'bumping into' Chris.
No accidents, they were all planned.
Introduce a little anarchy. Upset the established order