I feel guilty being thankful but isn't that what you're supposed to do?

I read about a mother and an 18 month old little girl battling cancer this morning and the mother's blog and the fact that the 18 month old little girl passed away yesterday.  It made me so sad, made me sob, though I don't know them, my heart breaks for them.  And then I say out loud, "Thank you God for my happy and healthy little girls".  I am so thankful that no matter how many times Hannah gets into the snack drawer and trashes it and climbs the stairs when I'm not looking and goes and steals something she shouldn't have and runs away with it laughing the whole time, she is healthy.  I am so thankful that no matter how many times Leyla has a breakdown and cries and screams over something stupid or says NO to me every time I tell her to do something or fights me over something, she is healthy.  It's times like this I question my faith, and I hate that I do that but I also know it makes me human to question things, but I question why God would allow a mother and a daughter to get cancer, for that father to watch his wife and daughter suffer, and then take their only child from them.  How am I supposed to be a strong Christian woman who my daughters can look up to when I question these things?  <BIG SIGH>  I know that God will never give us more than we can handle but sometimes I feel like he trusts some of us too much.  I love you Leyla and Hannah - you are truly God's gifts to me and I will treasure you always.  I hope that I can be a good enough mother that you can be proud of and look up to.