I think P10 is a nice test, I guess the only trikcky bit in it, is the canter across the diagonal for a young horse. I think that if you aim to get back to the track slightly before K or F it gives a bit more room as you need to come back to trot and down the centre line.
Canter across the diagonal and half 10m circle back to the track arent the easiest of movements but if ur entered your entered. Where are you going? Im doing 10 on sunday too!
It is a nice test and the first one I did with my young mare this year... however, on the canter across the diagonal just try to keep your horse as straight as possible and as someone else has mentioned, head for just before the marker to allow you extra room to trot and turn up the centre line.
P10 I think is the most popular test, it runs very smoothly. Saying that I hate it but only because its been 8 years since I last performed it and I can still remember the whole test by heart i've performed it that many times! lol.
Ah its a nice test and ur 5 YO should be fine as it was my 12YO ex racers first dressage test last week (only mastered circles 2 days before!!) and we came 3rd!!