I would suggest Legos. They have them in the basic colors and in pastels too.

My kids have always loved the "I Spy" books where you look for certain things on a page. I would suggest getting the ones for all ages.

I agree with books as a good gift.

I can't imagine trying to buy gifts for children I don't know. That's a tough one. Could you contact the children's mother and ask her for suggestions?

I was just at my sister's for her bday. She has 4 grandkids, 8 to 2 years of age. They love squirt guns. And if you have a problem with "guns", they had ones that you put the nose in the water, pull back on the handle and it sucks the water up into a big tube. Then push the plunger back down to squirt the water. Like drawing blood, haha, only about as big around as a kids wrist. Then push the plunger back down to squirt the water. They loved them.

Card games, board games, always fun. I played I think 50 games of Trouble, in one day, with my 7 year old great-nephew last year. He never got tired of it. Kids love it when you play games WITH them.

Here is a link that might be useful: water squirters something like these

Our grandsons love the squirt wands during the summer. We put them out on the patio with a bucket of water and let them get soaking wet. Play Dough and some of the sets of "tools" that are made to use with the Play Dough would be fun too. They also like the board game Trouble.

Take a look at the link below to fatbrain games it is specifically for 6 years old.
I prefer to give games that are brain stimulating and they have some good ones.

Here is a link that might be useful: Fatbrain

Rhizo, you have a lot of good ideas here. My grands have those "squirt guns" and love them. I like them because they don't look like guns. The Legos are a good idea too, but I like the "guns" better.

I too like the idea of "I Spy" books...Or Where's Waldo is age appropriate. Who doesn't just love Waldo?

Here is a link that might be useful: Where's Waldo on Amazon

Ravencajun, my grandsons love those stomp rockets on the site you list. In fact, I think they wore theirs out and are ready for a new one.

Sidewalk chalk is something that my practice grandkids love. They also love Nerf guns. You can get them that don't look like a real gun....

I would go to the bookstore and go to the children's section and find the activity books. The books that come with something that they need and then will be able to use that along with the book. Like the string game called cats cradle, something like that. I would get two different ones though, One for each child.

I thank you all so much for your ideas. I think that I'll hit some stores this week for a couple of fun things. I feel more inspired, now.

Sometimes just silly little things are entertaining for kids - we try to keep a basket of brightly colored flashlights at the beach, amazing how much fun kids will have with those in the yard at dusk and later when adults are sitting on a deck and end of the day is coming. They get to take them home when they leave, their very own flashlights ;)

Flashlights are always fun.