Michelle,
The ladies have given you terrific advice that is filled with so much wisdom. I agree that mathematical thinking is the goal and knowing more than one way to solve a problem is a wonderful way to show that kiddos are thinking mathematically.

So, it sounds like your kiddos are doing well in math. At age 6, there are no worries in having your daughter use her fingers (which are one of the easiest manipulatives to use because they're always with you).
As far as your son goes, doing the inverse of a subtraction problem to figure out the answer is a wise strategy. Either way, he still has to understand what he's doing and be able to come up with an answer that makes sense.
On a sidenote, If your son is still struggling with his math facts, you may at some point wish to go through a deck of addition flashcards with him and set aside any cards that he doesn't know within 3 or 4 seconds. This exercise will help you know how many facts he is truly struggling with. Then, if he's only struggling with up to 10-12 facts separate up that pile of cards and have him practice 3 the first week. The next week, add one more to the pile, so he is practicing 4 and so on, until all the cards are learned.
If, on the other hand, he is struggling with a great many addition facts, instead have him make a set of index cards flashcards for the 0's (i.e. 0 x 0, 0 x 1, 0 x 2, 0 x 3, and so on up to 0 x 12). Then, use masking tape on the back of the card to write the answer in pencil. Have him practice all 12 cards until he can get them down in 30 seconds. When he passes the 30 second test, move onto doing the 1's instead. You may wish to pay him 25 cents or 50 cents for each set that he passes. We did this with my oldest son (who did not use Singapore and did not know his facts either).
Just a thought.
Blessings,
Carrie