Mason bees are small, rather like hoverflies, and they don't sting. They are a major pollinator of flowers and need looking after, as numbers seriously declining.
A great project for the children, as well as the adults, and a major bonus for the bees.
You can buy this bee hotel anytime. Most are wooden and buying cheaper ones can see it deteriorate in a short time; plastic does best, but less attractive. In the end, the bees aren't interested in how it looks so long as it does the job.
Here, is a very easy way to make one or more, for a small cost. A length of downpipe, square or round, about 15 to 20 cm long; variations are fine. Double length means a bug house at both ends, so long as there is a divider (e.g. cardboard, crumpled paper will do).
Straws need to be 7 to 8mm wide and cut to length.
Ebay sell these jumbo straws. but ordinary straw will do. Pack the straw / straws tight. Don't forget to close the end up. Put the hotel outside in a sheltered place. (I used an old, discarded downpipe though any waterproof container will do. Even an empty paint tub, a narrow vase, an old wellie, even a Pringles tube (varnished, if outdoors) are possibilities.
Come the Spring you will have helped save a special bee and maybe some other pollinators.