Google - this year is my parents' 25 anniversary. both my brother & me are too young to arrange a?

Have you visited the e-How site (ehow gifts children 25th anniversary). Some good ideas there.
You are very much a part of this celebration, so personal things done & made by you both (including getting along for the day!) will count.
Other ideas could be:-

Make an anniversary card, decorations such as a banners,(in the form of silver letters (card ones covered with Aluminum kitchen foil).

Breakfast in bed for them. (best to knock on their bedroom door first!)

Make a scrap book (real or digital), of the family history of favorite memories.

Be their servants for the day!

Do you have any of your family, such as grandparents, uncle & aunts or even close family friends you can talk to or even help you (help with cooking a dinner or making more challenging things requiring tools? This could help you keep every thing a surprise by not risking things being found accidentally.

Hope these few ideas start you off & hope you all have a fun 25th celebration.