You need to look at the percentage rises in house prices in your post code.
The listing shows some but they cover several years between sales.
Some areas have a 10% price increase per year, others around 3%, others none (and even fall.)
I think you've increased yours by around 5% pa (maths experts correct me as I just did a quick mental calculation.)
I don't see why it's too tiny for 3 young children. You have 3 bedrooms, 2 receptions, a big dining kitchen and 3 loos, and a lovely enclosed garden.
It's far bigger than some other family homes.
Your family is only an hour away- okay they can't pop round at the drop of a hat, but I wish mine had been an hour away when DCs were small, rather than 6 hours!
An hour is nothing.
TBH I'd think about paying for help if you need it with childcare/cleaning/ babysitting- whatever- and save yourself the £thousands it's going to cost you to move in fees and tax.