I am really getting the feeling you really don't want the kids there if they cannot behave. Toddlers can be especially unruly as they have reached the age of independence and willfullness. Actually, most parents would like the occassional break from the kids and have an adults only moment. However, it's hard to tell one set of parents you can bring your children and tell another set you cannot. So it is an all or none situation and no in betweens.
You should NOT have to tell any parent to control their kids as that is the responsibility of any parent to be aware what their child is doing and how they are behaving. Especially at a wedding, because the moment is for the honored couple and not for the kids. So if parents cannot be courteous enough to control their children at a most, observed precious time, then they have no business in bringing their children to such events as this is to be the one and only moment to get married.
If those parents won't come because they cannot bring their kids, then they are selfish instead of realizing the special moment is for you and your groom. If they cannot set aside a moment of time to celebrate with you, that is their choice not yours, but if you are insistent in accomodating others instead of them accomodating you, then you have a few choices. These seem to be mostly family members' children you are concerned about, be vocal at both your bridal and wedding shower about how you feel children running loose, misbehaving, and possbily injuring someone or themselves at all these events each and every time you get together.
I would be open and honest with your "guilty" family members reminding them how Janie behaved at Joey's wedding, and Sara's wedding, etc. You don't have to be mean about it, but you can be frank and diplomatic at the same time. If you keep mentioning the subject, then hopefully the "aware" ones will get the hint.
Otherwise,
1. Hire a few sitters to watch the kids either at the place you are holding the event after you have confirmed if they will allow hired people to watch kids at their place (Liability insurance issues can arise). See if they can provide a separate room at a reasonable cost where you can hook up a few TVs (one to watch with some age appropriate DVDs and another to play video games) or choose someone else's place
2. Create a kids only table where perhaps a few adults will volunteer to supervise the kids table with fun activities for age range toddler to age 12. Coloring books, safety scissors and construction paper, origami folding competition, who can make the best tissue boquet (need glue and the bouquet holder can be a cone from construction paper) where you could have a moment sliced out for the adults to admire the work and the bride and groom to do the voting.
3. Or simply put, please no children at the reception as we plan to celebrate in style or specify a time the reception party will be adults only. This is a hint that alcohol may be present at the reception party and kids should not be a witness to the drunk antics of adults.
If you elect 3, again word of mouth at the showers as to why they will not be included because of the presence of alcohol is not a place for children to be about, that should settle the matter.
I did have children at my wedding. I had the activity table set away from the adults off to a corner so the kids could feel they were on their own. I checked every hour or so to make sure things were faring well over there and had a few volunteer adults take turns sitting with the younger ones to admitre their work. Also, I had the 8-10 yer olds to be "supervisors" of the toddlers to be "responsible" and play with them. They loved that as it made them feel a bit grown up. Any kids that deviated from the spot, I calmly grabbed their hand and reminded them that they could not be running about loose for their safety, Those that were getting a bit unruly, I took to their parent and told them I think Janie is feeling grumpy, antsy, or whatever the crime was and you may want to cuddle with her as my gentle hint get them under control. I only had 3 minor incidences of that and usually they calmed down once they realized they would be excluded from the kids activity table. The older kids pretty much did their own thing and were disinterested with the whole process and brought handheld video games to amuse themselves
We made sure the DJ had a few kids dance songs in which one set was for the kids only on the dance floor and then a few times where the kids could dance with a parent or an adult of their choice. We made sure it was done periodically so the kids didn't feel relegated to only the corner ALL the time as attention span and eay boredom is a huge factor for all kdis of any age. Also, we had kid friendly drinks and kid friendly food items at a kid only buffet table set up so they could help themselves without interfering with the adult foods (again with periodic adult supervision to make sure food wasn't getting wasted or getting out of control at the table).
As to my child, we always had a sit down talk prior to any special occassion. We went over the ground rules of behavior and expectations of courteous rules and the consequences if he failed ot observe them. Children may have a sponge brain for learning, but they need constant reminders of the rules as consistency is part of "practice makes perfect" or at least if not perfect, better learned behavior. He's age 10 now and I still go over the ground rules to remind him when we go out to special events together
Good luck, congratulations on your impending vows (1 year married and no regrets), and much prayers.