If it puts your mind at ease, I've never been to a buffet wedding that wasn't self-service. IME, people are generally fairly conservative in choosing from a buffet the first time through the line at a wedding, and I think it's pretty safe to only order half servings for kids under 7.
That's really nice that people are generally conservative in your area. That doesn't happen in my area of the Midwest. We have a fair number of over-eaters or persons incapable of determining portion sizes. Every large gathering that doesn't have friends volunteering to serve food at the buffet ends up running out of main items. And when I say every large gathering, I'm not talking potluck, just wedding receptions, graduation parties and (sometimes) meals after a memorial service.
If OP is uncomfortable on skimping on the food, I just wouldn't do it. My son could live on air unless he was going through a growth spurt. My daughter has an enormous appetite, even at a young age she would eat an adult portion of food.
OP, even if you have food leftover, if the thought of running out gives you any anxiety, don't do it. The extra money may be nice, but it's not worth your peace of mind with everything else you're going to have on your plate. Just my two cents! Congratulations on your upcoming nuptials.
I agree with the bolded. I think it's better to have too much food than not enough, and if it's a buffet I'm not sure how anyone could assign children's vs adults portions anyway. I know plenty of kids who can eat way more than me.
The worst wedding I ever went to was held at a very expensive golf club*. There were 9 tables of guests. For the main course, tables 1-3 went first and left barely enough food for tables 4-6 (I was table 4), so tables 4-6 had barely full plates. They then refilled the buffet for tables 7-9.
For the dessert, tables 7-9 went first, filled up their plates and left nothing for tables 4-6. They then refilled for tables 1-3. I don't know if there just wasn't enough food or those other people were simply greedy, but 20 years later I can still remember that reception for all the wrong reasons - being hungry!
OTOH one of the the best weddings I went to was a far less ritzy affair at a local bowls club and they were practically begging us at the end to go back for seconds/thirds as there was so much food. There were fewer choices, but what they did have they had plenty of.
* It was only a few months after my own wedding and we had looked at that venue for ourselves but got put off by the very unprofessional liaison person we met, so I was well aware of the prices.