Not sure about using Synths from the iPad, but this post links to videos which explain how to use the TouchOSC app on an iPad to send OSC messages to Sonic Pi.
I have made a series of three videos posted on youtube which show in detail how to create a simple layout in TouchOSC and set it up to control Sonic Pi 3. You can find the videos
here video1
here video2
here video3
or as a playlist here
The code used in the final Sonic Pi program (which is explained in detail in the third video) is below
#Test program to illustrate use of TouchOSC with Sonic Pi 3
#by Robin Newman, October 2017
set :trans,0 #set initial slider recorded position
#optional…