The Journal of Child Neurology has published research on the use of oral baclofen in children with CP for four weeks.  This study concluded 9 of the 10 subjects increased their voluntary control over muscles but overall strength (measured by plantar flexion torque) did not change.  The researchers hypothosize that the increase in voluntary control as a result of the baclofen may help to bolster strength training.

Reference:  van Doornik, Johan, Kukke, Sahana, McGill, Kevin, Rose, Jessica, Sherman-Levine, Sara, Sanger, Terence D.

Oral Baclofen Increases Maximal Voluntary Neuromuscular Activation of Ankle Plantar Flexors in Children With Spasticity Due to Cerebral Palsy J Child Neurol 2008 0: 0883073807313046

Check out Play Strong:  Activities to Promote Muscle Strengthening in Children Through Play at http://www.yourtherapysource.com/playstrong.html