"Out of the frying pan and into the fire" comes to mind! The fact that your child has taken the 11+ elsewhere is totally irrelevant in Bucks. The Bucks exam is VR only. The pass mark is 121 (roughly 86%+ raw score, top score 141 (age-standardised of course). The papers are NFER and there is plenty of advice on the Bucks forum from Patricia as to which question types are relevant to Bucks. Patricia or Mike Edwards may be able to advise you on how trafford and Bucks compare in content and difficulty.
Once you have moved into Bucks (and only then) you can apply for testing at any point in the year, or sit the 12+/13+ along with the herd. The full procedure is known as Late Transfer, and the guidance notes can be found here:
I don't know what the alternative to GS is in Trafford, but it isn't always very appetising in Bucks, so plan your move carefully!