I rent and self-drive all the time in Thailand, but you have to be comfortable driving Thai-style (i.e. not be a wimp). And of course, be comfortable with left-hand side driving with right-hand drive car. Major highway signage is usually in Thai and English, but off the main highways it helps to be able to read a bit of Thai, unless you don't mind getting lost from time to time. Never, never, never drive at night--I always get to where I'm going by 4-5 pm to find a place to stay (if freewheeling it) and get settled in. I also think major holidays that involve revelry are not the greatest of times to be on the road. Finally, I think you will find that you're not going to easily find a rental company that will do a one-way from Hua Hin to Krabi-endpoints are too far apart. Maybe for a humongous surcharge. I tried this a few years ago, and the best I could do was pick up at Surat airport and drop in Krabi.

Nobody has yet mentioned this, but one thing you could do, if just trying to get from Hua Hin to Krabi, is look into taking the train from Hua Hin to Trang. From Trang, you can get a bus, minibus, or car/taxi to take you to Krabi (2 hours max). The HH-Trang route you'd want is overnight, leaves between 8-9 pm, arrives Trang 8 am. Get a 1st class air-con sleeper. I've done this route (going the other way, and in 2nd class sleeper) and it's quite comfortable. I think the 3 yr old travels free. Tickets are bookable in advance (60 days out??) , which you should do given the holiday busy period. Much cheaper than renting a car, or a long distance car/driver. Once in Krabi, you can pick up a local car/driver by the day or half-day if you need to. This strategy eliminates the need for you to worry about driving yourself. For your reference: http://www.seat61.com/Thailand.htm#Southern%20Thailand