Should I take them all on? WWYD?

[Spixie33]

I don't see the problem at all.
21 month olds are not that hard to take care of and once they add another 6 months you will sometimes feel pretty bored watching them play because they will want to play ALONE most of the time with you just watching from afar.
21 months are also good at painting and colouring already so they don't need that much assistance like a 12-15 month old would.
I love that age and you also said they were PT so go for it woman ! It is not as hard as it sounds