If you want to understand why the Great Zucchini has this kind of success, you need look no further than the stresses of suburban Washington parenting. The attendant brew of love, guilt and toddler-set social pressures puts an arguably unrealistic value on someone with the skills, and the willingness, to control and delight a roistering roomful of preschoolers for a blessed half-hour. That’s the easy part. Here’s the hard part: There are dozens of professional children’s entertainers in the Washington area, but only one is as successful and intriguing, and as completely over-the-top preposterous, as the Great Zucchini. And if you want to know why that is — the hook, Vicki, the hook — it’s going to take some time.