If you trim the secondaries, it will slow the bird down but not hinder flight.
Secondaries and primaries should never be trimmed together.
Most clips advise trimming the primaries so as to prevent flight.
The cyan/light blue shows the "half clip" method. This allows for some flight if not trimmed too far down, as well as giving support for new feathers growing in. If the bird can still fly well, you trim a littler further back.
The yellow line indicates the "show clip". It's trimming primaries 1-7 and leaving 8 or 9-10 unclipped. It's supposed to make the bird appear "natural" and may, or may not allow the bird flight.
The green line indicates the coverts (or quill) clip. You trim either just below the feather lone of the coverts, or at the base of the feather near the skin (not too close, though!). With this method, you'll want to trim 3-4 of the outer flight feathers on each wing, then see how well the bird flies. If the bird flies well, trim another one on each side until the bird can glide down.
Have you ever considered flight training them indoors only?