Bicycle Tutor

I have a 16 year old Niece with learning disabilities with poor recognition of position in space. She loves going on her bike but can not manage to turn her pedals the full turn and gain momentum and usually ends up being pushed. Is there anyway the bottom bracket can be fixed so that the pedals will turn when the wheels are in motion and cut out the freewheel, just like a kiddies trike. She has a modern bike with stabilisers and I would be keen to know if this is possible?

If the rear "freewheel" is removable then maybe this could be replaced with a "fixed" sprocket. (See your LBS.)
If this is not possible then consider welding the freewheel to itself - but this will probably be a no return option.
You may be able to spot weld the outer of the freewheel to its inner. Then, if required, grind of the welds at a later date to re-instate the freeewheel.

To further cyclerUK's good suggestions:
- remove the freewheel and put on a "track cog". Get a bottom bracket lockring from an older 'cup and cone' style one and put that on on top of the cog. If both are tightened down, it should prevent the cog from coming unscrewed. This isn't ideal for a very strong rider, but sounds like it would be fine for your niece.
- Again if you're not talking about a super strong rider, you can glue the freewheel instead of welding it. Run some degreaser/solvent into the freewheel (try to avoid getting it in the wheel bearings.) Let it dry and then force some good epoxy into the interior of the freewheel. Not as strong as welding, but if you don't have access to a welder.

Thanks Guys
It worked an absolute treat. She was totally liberated and appears to have gained some independence.