Funnily enough, never come across this. My other half was glad for me to take our 7 year old as my first passenger as soon as my licence arrived. Wifey's a bit of a nervous flyer and to be honest I think she was thinking "thank god he's not taking me!"
My advice: Tell her you're taking the kid for a day at the seaside/theme park etc. and go flying - kid will love it so much they'll never let on to mum for fear of not being taken again - honest
If she finds out and divorces you - you can go flying whenver you want!
Seriously, the only people I've come across who had problems of this nature are a couple I know who have agreed to come with me seperatley, but not together. They don't want to leave several orphans behind if anything goes wrong. Can sort of see their point actually.