B.S.
Hi K.
The usual gift for this occasion is money. You usually put money in a money card. The amount of money us usually Chai or $18. Some people put in more than this but in a multiple of 18 like 36 or 72. If you want you can just give $25 or whatever you feel is appropriate for the child (how well do you know the child and the family).
The dress pants can be worn to the party. Just wear a dressy blouse and dress the outfit up with jewelry. If you are invited to the religious ceremony before the party, pants of any kind are usually not acceptable for a lady. I know all of this as I am Jewish and have gone to many of these occasions.
Have fun at this party.