It'll be cosy but as we motorhomed in a few 6m vans with young children and a dog, it will be fun. Watch the payload. It should be enough unless the previous owner[s] have gone to town with accessories. You might find that taking bikes will overload it unless you compromise with other items. Check the seatbelts are suitable for your young children - they are often not on the older models.
Also check the bed above the cab to ensure there is enough headroom. We preferred 5 berths with the kids because they had fights, talked all night, occasionally didn't feel well. This meant sometimes separating them at bed time. There is also a risk with two young children in the same bed that one will wet the bed.
Search the internet and you will find enough guidance of what to look for when buying a motorhome. Damp and loading margin are two of the biggest issues to think about. If your searches draw a blank, visit the Out&About Live website [motorhome section] which contains numerous guides for first timers.
Do some research into maintaining motorhomes because whilst the dream of getting up and exploring new horizons is very attractive, motorhomes have to be maintained. It's easy to see the attraction of motorhoming, but make sure you can cope with the reality.
I'd try looking for a 5 berth about 6.5m long as it will give you that extra space. Downside will be the loading margin will decrease. Not some much a problem if your licence entitles you to drive a vehicle over 3,500kg.
Auto sleepers tend to be reliable and reasonably well built although they are not renown for building family friendly motorhomes.
If you want any other recommendations, the give us a clue about budget, driving licence entitlement, and whether size is an issue. And good luck, you will find a suitable van and if the lifestyle s for you, have many years of great holidays.