There are four things we try to concentrate on during Lent: Prayer, Abstinence, Fasting and Almsgiving.
Try to pray a little more often, go to Mass more often, maybe attend a daily Mass, or just come by and visit the church after school one day. During Lent the Stations of the Cross (prayers about the journey Jesus made to his crucifixion on Good Friday) might be something to attend.
To abstain means to go without something; in terms of Lent we do not eat meat. Abstinence from meat is to be observed by all Catholics fourteen (14) years of age or older on Ash Wednesday, Good Friday and all the Fridays of Lent.
To fast, we go without food or eat less. It can also mean giving up a certain foods or treats. Fasting is to be observed by all Catholics who are age 18 to 59 on Ash Wedneday and Good Friday. Those who are bound to fast may take only one full meal. Two smaller meals are permitted, if necessary, to maintain strength according to one's needs, but eating solid food between meals is not permitted.
Do good things for others. For example, this can be giving gifts of money or help to others. Think about your time, talent and treasure. What can you do to help others?
A website with good information to review when explaining Lent to your children can be found at http://catholicblogger1.blogspot.com/2009/02/lesson-plan-ash-wednesday-lent.html
From the United States Conference of Catholic Bishops http://www.usccb.org/prayer-and-worship/liturgical-year/lent/
Explaining Ash Wednesday to your friends http://bustedhalo.com/video/ash-wednesday-in-two-minutes