I absolutely would NOT try it. We have never been to 1900 Park Fare when we have not had to wait a substantial time past our ADR to get seated. The first time we ate there, we had an ADR at 6:10. We wanted to go back to MK after, so we got there at 5:40, hoping we might be seated a little early. We were finally seated at 7:20 p.m. and we did not get out of the restaurant until 9:10 p.m.! If that happened to you, it would pretty much ruin your MVMCP night. We have been back a few times since then, and I think the shortest we have ever waited is about 20 to 25 minutes past our ADR. The characters can get really spread out in making their rounds, so if you want to see them all, it can be a long time before they all make it to your table. I have a 6 year old Princess lover too, and I would say that your time would be better spent getting in line for the Cinderella crew at MVMCP about an hour before the party starts (they will probably be a hotter ticket this year with Belle and the Beast being removed from the party- last year the line for the Cinderella group didn't really start building until about 35 minutes before the party- at least the nights we were there). At MVMCP you can get a photo with the group of Cinderella, Prince Charming, Lady Tremaine, and the StepSisters- the exact same crew you would see at 1900 Park Fare. The only thing missing is that you can't get their autographs at MVMCP since it is a big group photo.
Even on a good day, if you got seated at your ADR time, 1900 Park Fare is generally an hour and a half meal by the time all the characters make it to your table, so the best you could hope for would be to get back to MVMCP around 8, which means you would miss out on a good chunk of party time. You would be much better off trying to do a lunch TS somewhere that day, and then grab CS at MK before the party.