Creating a Block in Blockly

Well, you can easily do that, but you have to stack blocks inside of each other:

 after 1000ms do {
   set servo pos
   after 1000ms do {
     set servo pos
     after 1000ms do {
       set servo pos
    }
  }
}

I'm not sure that's actually such a hard thing to explain to children? It's harder for normal software developers because we're used to having things run linearly.

Failing that, there might be a way to modify blockly - but that's a question to ask the developers of that...