Yes, the invitation is for your little ones too! Now if you choose to tell them that they weren't invited to get a weekend away that's up to you.
Yes, please semi-formal attire is recommended. If you show up wearing white the wedding party will be pouring wine on you, so please do NOT WEAR WHITE. Also, it will be warm outside please dress appropriately. You do not have to wear high heals, wear some comfortable footwear we won't judge.