Hard decision, like you said, don't want to overspend if it's something they won't use for a long time. At those ages they will grow and need a new one soon. I would recommend your local bike shop more than department stores because they have pros that can help get the right fit which is one of the most important things. I like trek personally, but the base model bikes for most of the big brands are very similar.

Avoid suspension, 6/8 year olds don't need it and it will just add weight and cost. Same with 60 million gears, they use about 5. A heavy bike or one that breaks easily will make anyone lose interest fast. I've had many bikes from expensive suspended mountain bikes to dirt cheap steel single speed roadies. The fit, control, weight and quality make the ride and keep me going.

Kids love the freedom of having their own wheels and leaving you behind.