I know that this thread is old but I thought that people who end up here searching for ways to safely put kids in planes should know that Cirrus SR22 G5s do come with both ISOFIX and top tether strap anchors.
There is a seam in the top of the seat through which the top tether strap can pass. You then clip the tether to a floor-mounted anchor.
You can either use the three-point seatbelt or an ISOFIX system, if you have an ISOFIX car seat. Details are in the POH.
I fly with my one year old in his car seat regularly and it works really well. I do have the same problem as others, though, with him not wanting to keep his headset on.