Do they like characters? I would be sure to stop by the Toontown pre-party (starts one hour before MHP - so at 6:00 pm tonight - and runs for two hours - so ends at 8:00 pm). They have several treat trails (Minnie & Mickey's House, Donald's Boat, and one by Roger Rabbit) and character meet and greets (e.g., Mickey as Zorro, Donald as a Wizard). The characters themselves can be seen elsewhere during the day, but their outfits are unique to the pre-party MHP. If you do go at opening, I would start with the trails in the back (e.g., Minnie and Mickey's House) because the line seems shorter back there initially. The characters rotate (Pluto with Goofy, Chip & Dale with Mickey & Minnie, and Donald with Daisy when I was there), so depending on when you get in line, you may not see the character that's currently there when it's your turn (something to keep in mind if your kids have a favorite character).
Otherwise, there are lots of fun things to do at MHP with kids, but it really depends on what is your main goal(s) of MHP? characters? candy? entertainment? My group did opt to see PtN during MHP (we waited about an hour) because we had little ones and didn't want to try and make them wait 2+ hours (which is about when the curb/front row spots filled up on the non-MHP nights we were there). There is nothing special about PtN at MHP, but the kids still really liked it. If you little ones like fireworks, Halloween Screams is worth seeing. After that, we headed for the treat-trails and got a lot of candy. If you are planning on doing a lot of trick-of-treating, I would consider bringing/getting a larger bag to consolidate the candy. The little bags they give you fill up pretty quickly because treat trail has three or so stations and you usually get 2 or 3 pieces of candy per station.