Have just returned from a Thomson cruise to Jordan and Petra.
We had a fantastic but exhausting time. Coaches/motor vehicles can only stop at the top of the gorge, so you will need to walk or pay for some transport to the tombs. The walk is a mile, the terrain dusty and rough in places, but mostly in shade. The transport is a horse and cart with driver, very bumpy! From the treasury (the famous tomb) there is possibly another mile, but here you can pay for camel/mule/cart transport. Then you have to get back, and I think the transports will only take you parts of the way, except possibly the carts.
I think people don't realise how much there is of Petra to see, it just seems to go on and on! We (DH and I) walked it, but we were on an excursion so needed to be mindful of the time and get back for lunch at the five star hotel. We could have walked much more than 4 miles if we'd had a mind to.
There are people, including children, trying to get you to buy things everywhere, at stalls, and also walking up to you. Remember you have to haggle, it is expected! If you aren't interested, don't make eye contact, and say no thankyou. They will understand you. It will be a real eye-opener for your DD, seeing children her age selling things as a job!
Take plenty of water and a few snacks, strong trainers/ trainer style sandals, and a light jacket/ sweatshirt, if you arrive early in the morning there can be a chill wind through the gorge. There is a cafe of sorts when you get to the treasury tomb, but we didn't risk it!
Hope you have a good trip!