My DP and many of my friends are Sikh so have been to a few. All of the ones I have been to were very boozy weddings as alot of Sikhs LOVE to drink.
You often find that the weddings are quite extravagant too, a sort of show off within the community/wider family who has spent the most etc. If alcohol is involved you will most likely find that on each table there will be a bottle of whiskey/rum/vodka etc (often all of the above on average 5 bottles) plus all the mixers, non alc drinks for everybody to help themselves to. If the wedding is at the gurdwara and not at a separate venue this won't happen.
Check if you have been invited to the temple service and/ or the reception. As someone else has said the services go on for hours on end and if you have young children I'm sure nobody would mind if you don't attend this and just go to the reception. It is all about coming together and having a big party and there will be so many people around it is very unlikely that anyone will notice you weren't at the gurdwara.
Don't feel obliged to wear indian clothing. But if you do have the option of borrowing a suit from anyone then it is quite fun to get dressed up for the day, also female members of the bride's family really appreciate non-sikh guests making an effort.
If you don't then just wear something smart that you would wear to any other wedding. If you choose to wear a strapless or low cut dress then maybe add a cardigan or something as a sign of respect.
As for the kids, bring as many toys and colouring books as you like but hopefully you won't need them as they will be having far too much fun dancing, being paraded round on people's shoulders and dancing in glitter or balloons on the dancefloor.
You may find that you will not have much contact with the bride and groom at the wedding, it really depends. Sometimes the head table is sectioned off into a sort of VIP area. Sometimes they do the rounds to all the tables an say hi, but there really is SO much going on- I always feel sorry for the bride, she doesn't seem to get a seconds rest.
You will hopefully see them dancing though or doing the cake and photos.
Basically, don't worry about anything. Chances are it will be the biggest wedding you have ever been to (last one I attended was 600 people) there will be loads of other kiddies there and you will eat and drink so much you'll feel sick for days.
Enjoy x