I don't know if this will help in changing the DC diets, but I found this Easter recipe which might get a bit of fibre into them in an enjoyable way!
Mini Easter Nests. Makes about 12.
100g butter
2 tablespoons golden syrup
200g milk, white, or dark chocolate, broken
6 shedded wheat
Mini sugar coated eggs to decorate
Place butter, syrup and chocolate in a pan over low heat, stir until smooth and melted. Crumble shredded wheat and add to mixture, stir well, then leave to cool slightly.Line a muffin tin wth paper cases, put in the mixture and make a hollow in each to form a nest. Put in fridge to set before putting in mini eggs.
It's a long time since I made these, so I can't remember how big each cake was, you could use ordinary fairy cake cases and baking tins, in which case you may get more than 12. Unhelpfully the recipe says you can use any cereal and only gives quantity for shredded wheat, but Haggis could use branflakes if she wishes!
Flabbio, I understand you need to rely on the Movicol at the start, but sneaking extra fibre into his diet by whatever means will also help.