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.