I was going to ask you about this. You're stuck betweeen a rock and a hard place. If you buy tires for her, it's a slippery slope and she may start to hit you up for more things because they are kid related. However, it is a dangerous situation for the kids. Dunno.
Can you offer her a loan? Or buy her tires in lieu of some other expense that is further off so she can save up the money?
Since I don't have kids I'll defer to others who may have been through this.
If you love somebody, set them free. http://tinyurl.com/2empx2g