K in many parts of the world (including the USA) means Kindergarten which is usually age 5 going onto 6 years old. So, 1-5 means:
Grade 1: 6-7 years old
Grade 2: 7-8
Grade 3: 8-9
Grade 4: 9-10
Grade 5: 10-11
etc. etc.
It's hard to comment about Scratch, Lego Mindstorms and learning difficulties as it depends on what sort of difficulties and how severe.
There is a new version of Scratch called Scratch Jr available on ipads and free. It's geared towards K-3 children (kindergarten to grade 3). Code.org now has several levels of difficulty for their courses, including some for young K-3 children. Course 1 is the easiest. There is also Course 2 and 3.
You might want to start with those programs before moving onto full Scratch. After students are comfortable with Scratch, you should consider Lego We-Do Robotics which connects up to computers using Scratch software. It is more simpler robotics than Lego Mindstorms. The Lego Mindstorms uses a block-based visual programming system like Scratch, but it's different than Scratch. The Lego We-Do can use Scratch directly to program it.
It would be good for you to try from Scratch Jr and work your way up to see what each entails. Then, you can make your own judgement about the suitability for your students. Scratch Jr and Code.org do not cost any money. The We-Do and Mindstorms are an expensive investment and you need to know it will work for your kids before buying...
Hope this helps.
Vivian