We have a policy where birthdays is one or two large presents where as Christmas, just over a month later, is loads of smaller things, probably Sylvanian Families stuff this year.
You don’t have to justify your purchase to anyone here, none of their business.
For what it’s worth I’m in a similar position, but my girls are 5 and 8 – I’ve been looking at the Hudl from Tesco too but you can get the Kindle Fire HD from Amazon for the same price now (unless you’re using clubcard points or have a Tesco CC) and from what I’ve read online it’s a bit better. The Nexus 7 looks great though and if you can set up multiple users as said above that’s quite a handy feature.