Could someone give me some advice on weight loss?

[giveahoot22]

I admit over the past few years -mostly since my niece and nephew were born and I spend a lot of my time with them- it seems I have gained a few pounds.
Alright being truthful more then a few, the kids are 4 and 6 years old and they love the junk food my sister always brings in.
French fries, pizza, hamburgers, tacos, ect. And I can no longer stand it! I have gained more then ten pounds in a single year and I need some good advice.
I can not go on those strange diets that people are always talking about, nor do I want to. And I can't really do the jump around exercises they show on tv but again nor do I want to for it's so immodest!
So could someone just tell me how I can change a bit for the better without seeming like a fool while doing so?
I do a lot of activities with me niece and nephew and I want to be a better example for them, my sister just got back into church and has been filled with the Holy Ghost and I praise the Lord for that, but it seems she has yet to see the light on the damage of fast food she is doing to her kids.
So any help would be wonderful!
Because if it works for me then it could help them, my niece is only 6 but yet is 70 pounds! I don't want her to be made fun of because -due to my sister's work schedule- she is in *shudders at the thought* public school.
So I know her being raised as an apostolic will be one topic for her to face in school, I don't want her to be teased for her weight.
So again any help would be wonderful!