In addition to what everyone else has mentioned, road tape might be a good idea! And maybe a little construction vest and plastic hard hat for all the kids as a party favour?

Mini construction vehicles with food in them, sensory bins with dumptrucks, hard hats instead of party hats, wheel barrow for the presents instead of a present table, pin the hook on the Crain (or another part on another construction truck).

Have a look on Pinterest they have so many great ideas.
Like naming food, cakes, decoration, games.
My cousin had a construction party for her little boy a few years back, she made 2 mud cakes, one was decorated with diggers and trucks and the other was crumbled up to look like dirt.
Have a look at Kmart, Big W, Bunnings and Spotlight for things you could use as decoration and signs.

Borrow as many trucks as you can. Kmart have signs you could use as a work zone. Pinterest have food ideas. Decide all yellow and black, maybe red

Are you doing that cake where you create a chocolate bowl over top of the cake and hide lollies in there, then give you child a hardhat and hammer so he can smash the top in?

If you can find a park with a sandpit that would be good. Then you can bring some dump trucks and shovels that the kids can play with.

I think if you put some effort into the cake decoration, it makes a big difference. Otherwise I’d focus on taking some inexpensive toys (shovels, buckets), so that the kids can “construct” things themselves.

Maybe you can find construction themed songs and all mums join in. Or maybe make a big cardboard cut out of a digger as a photo prop

I’d go to Bunnings or my local hardware store for some decorating ideas/ supplies. Fluros, signs etc.

Trucks, sand, cardboard boxes seem like good choices for a construction termed birthday in the park

I am pretty sure there is plenty ideas on pintrest. Good luck

You can find lots of nice ideas on pinterest, for example here;
https://www.pinterest.com.au/PennyLaneS17/construction-party-decor/

Everyone gets one large construction block and have to work together to construct 1 big item.