I am having a BBQ/cookout for a wedding reception starting at 12pm in Sept. There will be 75 guests, 13 of them being kids 4-11yrs.

We were planning on having kind of a cocktail hour (pasta salad, fruit, chips, bean salad and some pulled pork)

Grilling would start around 1:30-2pm The plan is hamburgers, hotdogs and some steak strips and perhaps chicken.

How much meat for each item should we get? Or is this too much food? Trying to make sure guests have options and keep everyone happy.

Any help or suggestions would be greatly appreciated.

Your Dad is right on the timeliness issue; grilling chicken right takes almost 40 minutes, so if you don't start till 1:30...

I want to suggest some tweaks and rearrangements on your plan for ease of service and best quality food-

First, for the "cocktail hour", go with 3- 4 items, finger food type, NOT salads and sandwiches. If you put out pulled pork, everyone will scarf it as if nothing is coming later. My suggestion is,
fruit trays, see the fruit tray page, 1/2 the deluxe tray for 100 would be plenty
cheese 6 pounds, some could be logs or spreads and
cracker tray, 5 pounds crackers
2 great dips such as Mexican 7 layer (recipe on this site), humus, queso, etc. scant gallon eac of 2. 1 pound chips per 12; could be mix of potato, corn, tortilla, pita, depending on dips.
If you want a meat, do cocktail meatballs with toothpicks or stick pretzels. 8-10 pounds plenty, 3 quarts sauce

Second, fire up the grills and start grilling the chicken as soon as the cooks get there.

Definitely chicken. Budget permitting, whole chickens cut in halves or quarters, then cut into individual pieces once cooked. 8-9 chickens or or 34 leg quarters.

Second meat
steak strips- the difficulty with these is, if you have them, almost everyone will want some. You would need about 24 pounds.
An option would be good smoked sausage, about 15 pounds

Third meat; hamburgers, require a lot of extras, and not too party-ish. Take a look at this kefta kabob recipe instead, delicious and festive:

awellfedsoul.blogspot.com/2011/05/kefta-kabob-recipe-recipe.html

hotdogs- skip entirely. Kids will eat chicken legs.

Side dishes

pasta salad, 5 pounds dry pasta
bean salad, 2 gallons
Add a green salad (see plan for 1000 or marinated cucumbers or a broccoli cauliflower salad or slaw, 2 gallons

A wonderful addition to the menu of chicken and kabob would be a baked rice pilaf, you would do about 6 pounds dry rice, one very full roaster, or 2 roasting pans in the regular oven. Cut the pasta salad to 4 pounds. See the baked rice recipe in my recipe box.