These kinds of requests typically come for 2 x Primary School age children - say a Grade 2 and a Grade 5, so there is usually a difference in ability in English, just given their school experience. A 7 year old may have seen little more than vocabulary along with some basic structures, while the 10 year old will be working with much more. To be honest, I don't see that as a big problem, given that dealing with mixed ability levels is part and parcel of any group setting.

This is why long term teaching of a very mixed ability group often falls down, as they require different speeds of learning and certainly different styles of tasks to keep them engaged and motivated.

I would perhaps suggest if this is something you will be doing full time, maybe providing two different styles of worksheet to accompany the aim of the lesso; an easier more kid-friendly version for the lower level student and a harder more age appropriate one for the older, higher ability student. The drawback to this of course is double the work for you, I'm afraid!

Although, onestopenglish.com and cambridgeenglish.org both have some great resources that are level specific, so finding two different sets of material for the same grammar point or vocab group isn't as tricky as you'd imagine!

Good luck!