Alternatives to Vanguard S&S ISA?

I already have a S&S ISA with Vanguard, invested in FTSE Global All Cap.

I’d like to put away money monthly into a separate Vanguard fund or S&S ISA for my children, but still in my name. Vanguard won’t let me open a second S&S ISA.

Are there any similar funds within Vanguard that I could add approx £100 a month to, or a different platform where I could still invest in Global All Cap?