Instead of drumming-in the number area from One to Ten (and later further) now the rules for Null, One, Two - and utmost Three are sufficient:
Null and Null is Null
Null and One is One
One and Null is One
One and One is Two - but written as 10 - Never say 'ten'! You may call it "One-Zero"
One and One and One is Three - written as 11
(This "Triple Knowledge" will be used occasionally)

That's All!

or as a Memory table

		0	+	0	=		0	Null
		0	+	1	=		1	One
		1	+	0	=		1	One
		1	+	1	=		10	Two
1	+	1	+	1	=		11	Three

move on