Should we estimate legal fees at about 15k?
You'll make your solicitor very happy!!
Legal. C. 2k to sell, and another 2k to buy. Might be a bit more depending on title.
Stamp duty on purchase, 1%.
You'll also need building survey on purchase. Mate about 400. And you'll need a BER cert for you house to sell.
Estate agent, about 1.5% of sales prices, plus VAT. You'll negotiate with estate agent before you put on the market.
Budget for any small jobs / painting before you put your house up for sale.
Storage can add up if you've a gap between buying / selling. You've young children, so you'll probably have a bug declutter to do before you put house on market.
Do you have somewhere to stay, or need to budget rent?
Again, you might have some decorating to do in new house. And if you need to buy ant new appliances, etc., it all adds up.
I might have forgotten some things above.