Learning - Singlehanded or with Dad?

I would suggest a two pronged approach

they sail with you now in something stable and under-powered, the aim is for them to have fun.  Keep the sailing very short (less than 30 minutes).

When they get to 7 + (depends on child, earlier if they really enjoyed the sailing with you) you enroll them on a course with other children of similar age.  Between now and then you qualify as an instructor so you can help your club out (but don't teach your own children until they are beyond being teenage and preferably left home).

Sailing then goes two ways - they join the regional squad, national squad progress through the RYA scheme until they hit the olympics, reach their limit of skill or singlemindedness and sail a proper class or get fed up and discover girls (some of these options aren't mutually exclusive).

Alternatively they start racing with you and may or may not do some squad stuff. if they like the sailing who knows then where it might lead.

The not so good other option is they don't take to sailing.  When I joined families with the current Mrs P we had 5 children between us. 1 never went near sailing, 1 tried and decided not to, 2 got to stage 2 and my youngest son (21) regularly crews for me.

I agree that good crews are generally also decent helms. But just because you are a competant helm that doesn't necessarily mean you will be a good crew. I see youngsters who follow the Topper-4.7-Radial-Full Rig route and arrive at age 18 having never sailed in a boat with a jib or another person. That can provide its own limitations.

Both. Mine love mucking around in whatever they feel like that day (Snark, Puffin, Mirror, tiny canoe or something from junior group), either with or without adult company, but also  enjoy racing the Firefly, even though I'm not the most relaxed of helms.