Personally, I'd run away as fast as I could. Agreeing to do it will kill the friendship in resentment, anyways.
It would be a little different if your friend had come at this properly - researched the cost and duties of reception coordinator come to talk to you, mentioned that she needed someone to do this, had a detailed list of what was involved and time estimates, offered a fee, and asked if you were interested - she thought of you because she knew you were so good at things like this. You could think about it, look at the details of the job, and accept or decline as you wanted.
However, what she had done is essentially sign you up for major but nebulous responsibilities at her daughter's wedding and then told you about it, *after* she's told the venue you'll be doing this. She also hasn't said anything about payment, and I'd be willing to bet large amounts of money that she's expecting you to do this for free, with maybe a gift as thanks. This is not a job - it's a favour.
At a minimum you will be handling all the phone calls and communications from the venue, *and* 150 guests before the wedding, coordinating carpooling, giving out driving directions and organizing guests at the wedding (not to mention directing traffic), babysitting the badly behaved children, helping with setup, bringing enough ice for 150 guests, doing the site clean-up and packaging up leftovers, transporting and disposing of the garbage produced by 150 people.
I would also expect you to be expected to do things like shopping for supplies and running errands before and the day of the wedding, possibly arranging for the transportation of gifts, setting tables before the wedding, etc.
If the other guests don't stick around to clean up and transport garbage (in their wedding finery, with their carpools and no real parking) it will be solely your job. You will also expect to be doing several jobs at the same time - babysitting, directing people, set-up/clean-up *and* coordinating with the venue. Plus, your number will be given to *everyone* who needs to communicate abou the wedding - all 150 guests, plus the venue, plus the caterers, plus the florists, plus the photographer....
You are also doing this for someone who 'doesn't like problems' so if anything goes wrong it will be solely up to you to fix it.
So - I'd plan on 12-16 hours of work on the day of the wedding itself, plus 8-10 hours the day before, and a few the day after (going to the dump, delivering presents and leftovers). Count in 10-20 hours a week for a few weeks before the event, much of which will be spent on email or the phone. So expect 1-2 weeks of full time work equivalent, or 50-80 work hours.
I would charge at least $30 an hour, and track your hours. That would be somewhere between $1500 and $3000 for the event. I would also insist on hiring at least one babysitter for the children during the event, because you're not going to be able to corall kids with everything else you're doing, plus at least one other person for the cleanup phase. I would also insist on the rental of an appropriate vehicle for the wedding and the day before and after - a covered pickup truck or equivalent, so you aren't doing dump-truck duty in your own vehicle.