I am getting my kids new bikes and what i am looking for is a 26in bmx which is 7speed or can be converted to 7 speed. They will need gears a single speed wont cut it.
I am thinking 14/28 cassette with a 32 chainring.Possibly swapping the single freewheel for a seven speed. Claw type derailleur will do.
Hyd disc brake too.
Are there framesets i could build up.
And a 29er for myself.

Single speed 26" BMX cruisers and something that will take multiple gears are somewhat different animals. For one you will need a 135mm spaced rear end and hub/wheel to fit a multi geared freewheel or cassette. I might be wrong but most true cruisers are probably 110mm spaced like a BMX bike.

That's not to say what you want doesn't exist, it totally does but they generally get called dirt jumping mountain bikes. Quite a few companies made and still make multi geared 26" dirt jumpers. They will tend to have suspension forks though.

Thats what i was thinking.
Getting the wheel isn't an issue it's the spacing.
I have seen cheap versions of what i am looking for with rim brakes.
But i would like to do it right cos i want them too last a few years.

Might want to consider an IGH rear. Usually need less dropout width than derailer gear wheels.

I hadn't thought of them but i will have a look.
I picked up a mid ninties Kona superlight today which with some money and time fits the bill minus the disc brakes.