I agree wtih the two other wise mamas. I used to think "oh let them eat every time they want" so long as it's decent, etc. Well, enter my DD who humbled me and knocked me off my high horse

DS was the perfect child in terms of diet and everything else. He ate whatever I made at meal times/snacks. DD would live on white toast with butter, bottles and chocolate LOL.
I am not super strict but I'm also not going to "let" them eat however they want. Kids don't always know what is best for them which is why they need help. My DS is almost 6, and seriously can self regulate even sweets, but not always, and it's because we have healthy habbits and I teach them the why's and how's about what we eat. And disclaimer: I am not implying all kids will be this easy, nor am I suggesting you didn't do a good enough job if your 6 year old can't make the right choices...just giving my story....
We eat breafast lunch and dinner together, and i give snacks as needed. DS is like me, eats 2 or 3 times bigger meals, and DD NEEDS to snack. I don't give snacks while i'm making dinner. We have a later shcedule than most, homeschooling allowing so we don't get up as early, and eat dinner at 7. I always toss the salad or have a veggie ready a little early incase kitchen nibbblers come to help, it gives an extra something healthy. They love to set the table, and I let them do it with whatever/however t hey want. Since stocking more tablecloths (all washable, nothing too spendy or fancy) and creative dishes they enjoy dinner time because they get to make the table setting.
I DO NOT keep an open pantry system! The chips/sweets are separate not visible for DD's sake, and we prepare most everything we need to eat. I keep cheese chunks, veggie sticks and yogurt that is available on their own, and always have homemade snack bars that are good for you. DS is 6, and likes to help prepare his meals, and DD likes to help with hers too, and this helps a lot.
We talk about meals together. I ask everyone sundays about what they'd like to have during the week, and I incorporate something everyone really loves every meal that I know. If DD is on a pasta kick, I make a TINY amound of a dish and then she can have more of something else.
Avocado chocolate pudding and smoothies with cooked carrots/squash are a couple favorites of my DD.
I try to be as gentle as possible with the let downs. I just cannot see them going through the weight problem I once had. Just because they are thin now, it doesn't mean a junk diet is gonna work in the long run, and it's hard to change habbits later.
I have become really passionate about kids fitness/eating. I even know moms who are obsessed with workout and staying healthy and make fabulous food for themselves and feed their kids fast food junk.
Like i said I used to be on a pedestal big time. SOme people still are. That or they'd rather give in to every single whim and never ever have an upset child so they convince themselves it's for the greater good to make the same three things time and time again and be a short order cook all day and night. And I DO do that to a certain extent...the short order cook part, but it's cause i like to cook and they usually ask for something interesting and i don't mind making. Dinner is another story and like I said dinner is dinner! Especially since we know ahead of time what's on the menu. I also am pretty good at making things different ways and don't mind. If i'm making meatloaf which we all love, or a shepherds pie, I make meatballs for DD out of the same thing and give her a SIDE of potatoes and make her shep pie filling look like soup with a little scoop of shell noodles. Happy camper and everyone is eating basically the same thing
boy this became long winded LOL.....just random ideas here