1v1, 2v2 small sided games will help with this. Also praising the slightest positive will also help as everyone likes hearing praise. Using the word "try", and praising when they try something you ask them to do. The difference is, if you ask them to try to make a tackle and they do try (even if the tackle is unsuccessful) - they have achieved exactly what it is you have asked them to do and as such - will have a positive to take out of the moment.

I have this same problem with our team. We have some really technically good players that are now doing less running and losing confidence despite lots of praise. The less technical players are growing in confidence and getting technically better but the others going the other way. 

Its part confidence but also they have just stopped running with and without the ball. Instead of running on to a loose ball and running in to the space, they are now running up to the ball and stopping and then getting tackled. Off the ball they Jog and getting run past.

We played a team last week that we dominated 6 months ago, and this week we lost 10 nil and had one shot. It's not about the result however the difference in our team was Incredible. 

We will continue with the praise and I'm sure it will come back but any drills to get them running would be good. 

I have an idea that you can promise the players with some gifts or prizes if they play with enthusiasm and become positive in the game and become not afraid to play as they want and try new things and try for play for winning , you can give them gifts such as footballs or t-shirts or pen or pencil etc

Try to build competition with the 1v1 practices. You can also build it within their skill development by adding levels to the skill. For example, if all of them are right footed, challenge them to try dribbling on their left only.  That's level one. You can also talk about different parts of the feet when dribbling to them which is another level and then finally if they can lift their head whilst dribbling with the ball

sometimes in my experience continious praise can lead to a negative, try first praising but along side that asking why, what, how.

can you try this today.

this essentially could have the same end result but will vary the way they apply it. 

making it more interesting and exciting.

the other thing to consider is they are 7yrs old their young minds wander, give them 5 mins on the side to focus on something whilst someone else has a run round with their mates