For engaging players to try to get back on breaks by the opposition, work on a small sided game in practice. Try to position players where they are likely to play in the next game.

If you have a 6 a side game, for example, include a condition that if four of the five outfield do not get back into their own half, any goals count double.

You will probably get one or two who may hover near the half way line and step across to avoid the forfeit. However, if they can do that, get them to engage more quickly and make the earlier run to get back more quickly. 

You (or a parent) can measure this in a game as well as practice but you will need to observe the reactions of the players around and away from the ball when your team loses possession, so try not to watch the ball.

Q. Why are we doing this coach? A. if they attack our goal and the GK saves or the shot is blocked we need to have players ready to pick up rebounds and not let the opponents have a free shot. Also, we get back to put pressure on someone who may receive a pass and therefore delay or snuff out the attack.

For the slow player, ask them to anticipate where the ball is going and to get on the move 'earlier'. Also, take 2 - 3 small quick steps to get moving quickly and then lengthen stride as they get going. You may not be able to make them faster across the ground but if their reactions are quicker they will get where they need to be better than they do at the moment.

Finally, depending on your team arrangement / formation, does the first defender / midfielder always try to win the ball? 

If so, set them a condition to 'jockey' the opponent - near enough to close down, but far enough away to avoid being beaten. Try to guide the opponent with the ball towards one (the nearest) side line. Try to prevent a switch of play and try to delay the attack for the others to get back. Hope this helps