We investigate the conditions for development of Kelvin-Helmoltz instabilities along a collimated wind flow from a low-mass star surrounded by a dense circumstellar disk, which could give rise to condensations with the characteristics of the chain of HH knots in the HH 7-11 system. The flow, using the observational parameters of the source, turns out to be clearly unstable. We give a simple estimate of the instability time scale, which turns out to be in good agreement with the characteristic time of development for outflows from low-mass young stellar objects.