I found a way they could work together to achieve both goals.
We ended up exploring water pressure at the same time. Because the higher we lifted the funnel end, the more pressure we gained and the faster the water came out of the other end.The boys organised themselves into taking turns, pouring and holding, and soon were exploring the effects of raising and lowering the funnel.
I had seen a children's science programme, in the holidays, were a man holding a pipe and funnel had used the water pressure to lift a person standing on a hot water bottle, attached to the other end. They'd even burst the hot water bottle. So once the bucket was full I asked the boys did they want to try blowing up a balloon. They did and with a rubber band to hold the balloon onto the hose, we gave it a go.The results were spectacular, noisy and a lot of fun. We repeated our experiment several times, recycling the water (we have a drought.)
Loving Balloon volcanoes in the sand pit already (which we usually do with the hose and tap), we decided to try it with the hose and funnel the next day.
Soon more helpers were recruited. One boy found lowering the funnel now could result in a reverse flow as the sand compressed it. We all watched as the balloon swelled and shrunk as we raised and lowered the funnel. It was a real group effort, as some filled, some held, some dug the sand to cover the balloon and some just watched with interest. All the while we were learning about cooperation, coordination, water conservation, patience, water pressure and volcanoes as well as expanding our vocabulary and concepts around these things. I guess we really should have counted how many buckets it took and measured the volume as well.It worked and worked spectacularly, sometimes leaving a perfect crater lake (something the tap method doesn't do.) And always going off with a bang. (I think we'll use a rubber band even with the tap and hose method from now on.)We repeated it several times as we emptied the water trough for the day and even got it on video.
Great fun, give it a go too!